In the modern era of guerrilla warfare, the use of ideology became one of the prime movers of the masses. Soldiers were recruited through appeals and introduction to ideology through simplified means, whether it be to a nationalist liberation ideology or a Marxist ideology. However, how do you mobilize and recruit the masses in a fast and efficient way when 90 % of the population are illiterate? Going around from hamlet to hamlet holding speeches and lectures would take too long. It is far more efficient to print up a few million pamphlets and distribute them to the people, right?

This was the task that the Viet Minh under Ho Chi Minh found themselves with when they in September 1945 (a date that some historians argue is the actual start of the French-Indochina War as opposed to 1946) launched a literacy campaign to turn the peasants of Vietnam into reading peasants. Recognizing the challenge at hand and adapting themselves to the first tenant of the Maoist guerrilla theory (mobilizing the masses and making them politically aware), the literacy campaign in the areas controlled by the Viet Minh became hugely popular. Historically, literacy amongst the rural population had been prevented since there hadn't seemed to be a need for the peasants to be literate, effectively turning the ruling classes into the only ones able to properly read and making the rural population easier to control. For the rural population under Viet Minh control in the late 40s, this was a complete and utter break with what had come before and elevated the average peasant to the level of those who had once stood over him.

In the context of guerrilla warfare however, this campaign played an enormous part in gaining the trust of the rural population. The Viet Minh would be the ones arranging these classes, going from village to village and thus making the insurgents a consistent and positive presence in the daily life of the peasants. It would gain the Viet Minh a direct contact with the individual and would hence be more willing to join the Viet Minh. The individual came to play a big role in actually spreading literacy. When the Viet Minh was not available, newly literate individuals could teach their families and friends in turn. The classes themselves were more than not adapted after the rural population's lifestyle and agricultural schedule and those who were reluctant to join were, in the words of historian Martin Windrow, encouraged by peer pressure. These classes naturally became breeding grounds for indoctrination, which is precisely why they were carried out to begin with, yet this indoctrination was combined with other useful and helpful features for the average peasant. One organizer described one class in the following words:

We put them into groups of 20 or 30 people. Besides teaching them to read and write, we also helped them with their family or economic problems. For instance, if the house of a student has burned down, then the whole class and the whole unit will devote its effort to helping that particular student. Therefore the students are responsive to the class: they are eager to attend the class because the class is not only where the people learn, but also the place where we discuss other problems of collective interests.

The campaign became a fundamental part of Viet Minh recruitment. Beyond what I've already mentioned, it is worth to point out that through the campaign that was very much aimed at the young (even though there were no exclusions: age and gender did not matter), the Viet Minh got a base of highly motivated, grateful and inspired recruits who saw the senior soldiers and officials in the Viet Minh as men and women they could trust and who even took over their traditional role in their rural communities while they, the fresh recruits, were out fighting. With more of the population able to read, the Viet Minh was quickly able to spread the words of their ideology and turn the important rural population to their side in their fight against the French and their Vietnamese allies.

The campaign was a huge success. According to Viet Minh sources, illiteracy went down from 90 % to 20 %. The campaign would go on to inspire several other literacy campaigns in North Vietnam and later the reunified Vietnam. All in all, according to Phan, Bilgin, Eyland and Shaw in Literacy in Vietnam—an atlas (Asia-Pacific Research Institute Macquarie University, 2004), Vietnam went from a 10 % literacy to a staggering 90 % literacy and it all started with this particular literacy campaign.

Sources:

The Last Valley: Dien Bien Phu and the French Defeat in Vietnam by Martin Windrow (Cassell, 2011)

The Rational Peasant: The Political Economy of Rural Society in Vietnam by Samuel L. Popkin (University of California Press, 1979)

And now the Early Modern companion to /u/Bernardito 's Modern answer!

Recruitment:

There were three primary means: commission, compulsion, contract. Authorities give commission to particular persons or parties and assign the areas of recruiment. The recruiting officer, effectively the captain, names his officers and they then go visiting towns and villages. They coordinate with local authorities and put up a show for the locals. Flags are put up, drums beat, and the captain then screens the recruits who show up. They are paid an advance, given food, shelter, and eventually clothing. So only the captain had to be literate, and he then leverages displays of heroism to aid in recruiting.

Compulsion is simple: offer to prisoners the prospect of earning their freedom. Contracts are what is often called "mercenaries". The captains are paid a contract to bring a said number of able soldiers.

Training:

Tools were starting to be developed, and a major development was in the Spanish army. They had training camps in Italy that they liked to use for green recruits, where they stay a few years. Then they are sent to the Low Countries where they typically served in garrisons. Finally, they are considered sufficiently experienced to be front-line troops.

Regiments have Sergeant majors and training camps, and illustrated instructions such as De Gheyn's Manual of Arms: Pikes and Manual of Arms: Muskets.

The Sergeant major and his assistants trained and drilled soldiers in various arts, and they are responsible to assist the captain to ensure good formation during battle as well.

So, the Early Modern era was the era where mass recruitment and conscription changed the nature of armies, and thus systems and tools were developed to overcome the challenge of illiteracy.