To specify, what I mean is that Shinto traditionally is a very diverse belief system, but State Shinto was a uniform belief system that was the same everywhere in Japan, unless I am misunderstanding it.
For it to be uniform the state must have clarified what it considers to be Shinto traditions, beliefs and practices that everyone must follow, and which aren't (any longer). So what did that look like and how did it differ from normal Shinto beliefs?
Let me give a more in depth response since I'm no longer on a device that keeps malfunctioning.
Part of this question is a matter of understanding some of the details of Shinto practice in Japan, especially before the Meiji Restoration. Shinto before the Meiji Restoration was a distinctly local religion. Shinto priests were not appointed in any standard fashion, but instead were "independent sacerdotal lineages managed by a small number of the largest shrines.(Hardacre 5)." Basically, different communities practiced local rituals and had different standards for what the duties and requirements for the priesthood were. Shinto holidays were highly variable, celebrating seasonal rites for local kami involved different practices and different dates. You would celebrate the harvest at very different times in Kagoshima vs Iwate.
The Tokugawa Shogunate of that preceded the Meiji Restoration was nominally Buddhist, and even major Shinto rites frequently involved Buddhist priests leading pilgrimages or undertaking austerities related to the events.(Hardacre 13). Most important to understand is that the term Shinto wasn't really used in ordinary speech. It became a category for the practices of different shrines during the late Shogunate and the institution of Kokugaku, often translated as National Study or National Learning. The Kokugaku movement sought to create a unified Japanese identity in a way similar to the use of folklore to create a unified German identity in the 19th century(see Cultural Nationalism: The Grimm Brothers' Fairy Tales by Louis L. Snyder). Part of this was a distinction between Buddhist rites that were seen as related to practices in China and Korea, and rites that were distinctive to Japan, which were categorized as Shinto.
During the Meiji Restoration, the Meiji oligarchy(the emperor was a young teenager during the restoration) sought to remake Japanese society to establish the legitimacy of their government. Part of this was overturning class structures that had been the center of Tokugawa power. The Charter Oath of the new government included five provisions. 2 and 4 are key to understanding the process.
2. All classes, high and low, shall unite in vigorously carrying out the administration of the affairs of state
The "unity" of the classes in the affairs of state shows the efforts at national identity around state practices. The "evil customs of the past" included the influence of the Buddhism on state culture. These were the underpinnings of State Shinto as it developed during the next few decades.
What State Shinto looked like was essentially a creation of a hierarchical bureaucratic structure of Shinto religion, centralized around the Ise Shrines. All Shrines were ranked somewhere in the list and the nation was essentially divided according to local shrines. The people of Japan had to register with their local shrines as ujiko, which became the official census of the government. State Shinto was as much a state bureaucracy as a state religion, though it's important to realize this is not that different than practices in contemporary European nations, where parish registration was a common form of documenting births and deaths. (Hardacre 28-29)
The practice of Shinto on a local level did not necessarily change that much. Shrines still celebrated local rituals. But there was also now a centralized religion that was dedicated to the glorification of the Emperor as a divine figure. This involved the construction of a centralized Shinto including the creation of Yasukuni Shrine in 1879, which enshrined war dead and was framed as a high national honor. Overall, this bureaucratic structure was devoted to the glorification of a certain ethnic nationalism that was one of the underpinnings of Imperial power until the end of World War 2.
Ultimately, it's worth recognizing that the existence of "State Shinto" is not an official practice, but a later description used to categorize practices that the US sought to undo as part of the aftermath of World War 2.
Hardacre-https://books.google.com/books?id=v-E9DwAAQBAJ&pg=PA7&source=gbs_toc_r&cad=3#v=onepage&q&f=false
I want to expand upon what /u/drydem posted and talk about the formation of State Shinto (kokka shinto 国家神道 or kokutai 国体, the "country's body"). There's a lot of politics involved and plays for power that are pretty interesting. In some ways it begins with the idea of religious freedom. From the late 1800's through the early 1900's there is a bit of back-and-forth between what the State (imperial court) supports and what it does not. Keep in mind this is all a bit complicated because of how "shinto" may be defined. More on that later. For now, let's consider the idea of "State Shinto" itself.
As Nitta Hiroshi and Yasumaru Yoshi posit, "State Shinto" is a bit of a burdensome phrase because it implies any historical context that don't fit the mold of "state" or "shinto" ends up distorted. The phrase's origins and popularisation are based out of the American occupational army in 1945, purporting to abolish kokka shinto in order to affirm freedom of religion and eliminate militarism/ulta-nationalism. I think to this day there is debate about the term, with Hardacre prefering to use it while others like Teeuwen choose not to, saying modern shinto history can be covered without its use. This is because, as Hardacre points out, State Shinto is frequentyly coupled with the "Emperor system" (tennosei 天皇制) to the point where there's a perception that the State made shinto its "puppet" which included extension into education and other civil aspects of Japan. To use Yasumaru's words, the word ("State Shinto") came before the meaning.
This isn't to say the term kokka shinto didn't exist before 1945. Rather, the meaning behind it was different. From the perspective of what it was perceived as in 1945-
Put simply, Japanese leaders in the Meiji era (1868-1912) created a new state orthodoxy centered around a sacrosanct emperor, who was to be worshiped by all subjects at government-sponsored shrines in a system of State Shinto. Once established, according to this interpretation, the emperor-system state dealt harshly with the new mass religions that worshiped deities other than those in the officially prescribed pantheon, as its guardians freely invoked the charge of lese-majeste' to crush groups considered heretical.
Compare this to a statement by Oda Kanichi to the Diet committee deliberating government handouts to shrine priests in the early 1900's
In 1882, the distinction between "religious shinto" (shukyo no shinto 宗教の神道) and "state shinto" (kokka shinto 国家神道) was already clear enough to behold...
Therein lies a key component to this discussion. Contemporaries at the time were viewing shinto from a religious and non-religious aspect, with the non-religious aspect referred to as "State Shinto." This seems to diverge from the previous quote about a sacrosanct emperor and "heretical" groups.
To find out why we need to go back further to 1872 for that's when the Jingisho (神祇省 Ministry of Shinto Affairs) was abolished after less than a year in favor of a more "encompassing" Kyobusho (教部省 Ministry of Religions). Note the Jingisho was itself a replacement of the Jingikan which goes back to the early days of Japan. The Kyobusho was a program of "edification" spearheaded by shinto, buddhist, and confucian propagandists designed to address failures of earlier shinto-only edification programs. At the same time, anti-Christian notice boards were removed to play nice with Western influences (this was Meiji after all). Both these actions were taken to mean that the debate about religious freedom was being given an opportunity to be hand.
The reality was a little different. By late 1872, the Kyobusho was being increasingly swayed by anti-buddhist influence from Satsuma bureaucrats. This resulted in the Daikyoin 大教院, a privately funding body backed by the government to coordinate messaging and serve as a place for cross-sect discussion on issues, being effectively "hijacked" by the pro-shinto lobby. To those of the Shinshu Buddhist sect, it was a plot to convert buddhist temples into shinto shrines and buddhist clerics into shinto priests. Shimaji Mokurai was a prominent Shinshu Buddhist who took the lead on extricating buddhism from this control. And his path to freedom was the idea that shinto was not a religion. His associate Ouchi Seiran submitted a petition to the the Sain (左院 lit. Department of the Left) that began with-
What we know today as religions, Buddhism, Christianity, etc., are all universal...
Notice shinto is left out. He addresses it later-
If you insist on calling this shinto a religion - we should really call it not merely a polytheistic religion, but a rag-bag religion... Shinto riturals are national or public in character and so the state should itself perform rites at national shrines; central government should ensure that local governments perform local rites and require the participation of local people. If however we are to regard Shinto as a religion, it then becomes a question of the people's freedom whether or not they believe. You can try all you wish to make people have religious faith, but if the people do not have it of their own accord, all your efforts will be in vain.
The argument is that religion and state combined is a concept from times past, and that the state must distance itself from religion. However, it must not distance itself from ritual. The argument for religious freedom was a matter of benefit; by separating shinto ritual from religion, it created a separation of buddhist and shinto administrative controls. In short, buddhism would be free to proselytize and promote its teachings unhindered.
Meanwhile, shintoists countered with a need to resurrect the Jingikan, unify religion and state, and ensure that religion is Shinto. Tanaka Yoritsune, Chief Priest of Ise Shrine, likened the association of religion and state to the Pope in Rome where-
... state law and church law are indivisible. Government and religion are one and the people, as a result, support and revere the sovereign.
The State was caught between both arguments. By 1875, they had shifted alignment towards Shimaji's idea and ended up dissolving the Daikyoin. The Shinshu Buddhists campaign was a success. The State was no longer susceptible to arguments for proximity of state and religion, and the idea of "shinto as non-religion" became a powerful weapon in the hands of those seeking to gain from shinto's split.
One important thing to keep in mind with the Shinshu victory was that they declared their intention to accept Amaterasu (the sun goddess, main kami of the Ise Shrine) as the imperial ancestor while simultaneously making her integral to Shinshu Buddhist teachings. This veneration of the Emperor likely helped the State with the shinto as non-religion problem. You might see where this is headed.
Before we get there, we have to go through rebranding. With the Daikyoin's abolishment, shintoists almost immediately setup the Jimukyoku 事務局, or Office of Shinto Affairs... and fell into disarray. The infamous "Pantheon Dispute" (saijin ronso 祭神論争) erupted as to which kami should be housed in the Jimukyoku. It was ultimately decided to ask the Emperor to approve proposals of acceptance. It was also during this time that the need for a propaganda office emerged. Priests called for a new Daikyokan 大教館 to be established in government, headed by an official senior to the Prime Minister who exercised supreme control over religious matters, assume responsability for court ritual, and basically be in charge of all shrine priests, propaganda activities, etc. That included drawing up rules and regulations for religious activity. And they did this through the notion of Shinto as non-religion-
The fact that the words for both "ritual" 祭 and "government" 政 are read matsurigoto in Japanese constitutes a great teaching, a constant reminder of origins. This teaching is the source of all the teachings of the world. This teaching has been inherited by emperors over the ages and there was never a change to its substance. Consequently, there is no distinction between deity and emperor: the Way of the kami (shinto) and the Way of the emperor (kodo 皇道) are one and the same. Here lies the essence of Japanese teachings; all ethics spring from here; the foundation of the kokutai 国体 lies here...
The argument was that shinto was non-religion because it is synonymous with the Imperial Way, thus making it impossible to view it in the same category as other religions. Shinto's rebranding had to push shinto as non-religion due to the previous Shinshu campaign, and it did so by saying it was one with the state, a "supra-religious entity" (to use Teeuwen's term) above all others.
Naturally this didn't play well with Shinshu Buddhists, Christians, and other non-shintoists. They claimed shinto was behaving as a religion with its propaganda activities, and thus called for the abolishment of the entire kyodoshoku 教導職 system- the appointment of members to the Kyobusho mentioned above. They also called for anything that made Shinto appear as religious be prohibited and separate the government offices having jurisdiction over shinto rites from those responsible for buddhism matters... while allowing buddhists to preach their own teachings.
(continued below)