I'm going to put aside the term "dark ages" and provide an answer from the early medieval period, prior to the rise of universities, which changed the system significantly. My major source for this (other than the course in palaeography I took many years ago) is Raymond Clemens and Timothy Graham's Introduction to Manuscript Studies. (I am also an amateur calligrapher and illuminator)

If you're a monastic scribe, you are likely working as part of a team of scribes to copy the text. Most books were limited in decoration, but large Bibles or Gospels that would be used in a church for regular services were those most likely to be decorated, and most frequently produced for high-status clients such as bishops or high-ranking abbots. There were also simpler books for everyday study or use within a particular monastic house; copies might be made for a daughter house or another house of the same Order. The vast majority of the books copied in the Early Middle Ages were biblical in nature, as these are the types of books most likely to be in use within monasteries (for example, as lectionaries--collections of readings the monks would hear on a daily basis). Monastic houses might also compose and/or copy saints' lives, particularly for locally-venerated saints (for example, St. Cuthbert at Lindesfarne) During the "Caroline Renaissance" copies of Roman secular authors and philosophers also began to be made more frequently. The heads of monasteries often had a big influence on what kind of books were copied or held by the religious houses under their control.

What was life like? Per Clemens and Graham: "Copying a text was a generally lengthy, laborious work, often carried out in demanding conditions (insufficient light, cold that numbed the fingers, etc.)" Monks also had their regular duties in attending services, and Clemens and Graham estimate that the average monk could likely only pursue scribal work for about six hours a day, about 150-200 lines of copied text. Text was almost always copied first, with space left for decorated initials or larger decorated pages. It's not known for certain whether in this period both the illumination and text were done by the same scribes, but it is quite likely they were done by different people.

As for the style, there were particular styles that were in fashion in various times or places--for instance, the 8th century Lindesfarne Gospels and the 9th century Book of Kells, both products of the British Isles, are both in a style known modernly as "Celtic Knotwork" and also share similarities in script. The person writing the text would usually leave some sort of indication of what letter was needed in a particular blank spot. Beyond that, and beyond adhering to the basic decoative language in current use and palette of available paints, the illuminator likely had a certain amount of liberty, in keeping with the subject manner of the book being copied--for instance, the use of the symbol of an eagle with the Gospel of John, or a lion for the Gospel of Mark Illumination during the early medieval period is generally quite stylized (especially in the British Isles), so we are not talking about detailed scenes meant to be realistic depictions.
For the monastic copyist, scribal work was true labour, but it was certainly considered hugely important work to the daily life of the monastery, as the books produced were extremely valuable and in regular use.