Short answer: yes, there were precise instructions and models to go on, and while there wasn't exactly a 'standard book,' scribal work was a monastic tradition that was part of everyday life and was taken very seriously.
Longer answer: the material culture around books (how they were made, who made them, who physically produced them, patronage, etc.) is a pretty trendy topic right now, and we know a great deal. First, know that bookmaking in the middle ages was a very, very expensive process. Part of the reason literacy was low was because only the church and the extremely wealthy could afford to commission and buy books. The parchment is made of animal skin (usually sheep or cow) stretched out and treated in a very involved process--and since the skin had to be as close to flawless as possible, these had to be high-quality animals, which incurred even more expense. A given book is made of dozens, sometimes hundreds of animal hides (one extraordinary example is a book that boasted 600 head of sheep). After that, consider that ink is expensive, including the colorful ink and gold leaf we see in some of the prettier illuminations. Bindings were more flexible, because they could be replaced, and often were, when the book changed hands. In fact, we see instances where the parchment is cut at the margins to fit a different binding than the original.
Cost and production aside, yes, monks (and later, professional scribe workshops in cities) has very precise methods for their work. They'd first use a tool that looks like a serrated pizza cutter to make tiny, evenly spaced pinpricks along the edges of a book in order to make lines to guide their hands. You can still see those lines in some books, but they're usually kept subtle or erased in the nicer ones. Then everything has to be planned out before actually going on the pages. I know low-quality or scratch parchment was sometimes used for this, and I think clay, but I'm not sure. Then the monk/scribe will start writing, quill in one hand and a scraping tool in the other (on animal skin, you 'erase' by scraping off thin layers of ink). In order to keep writing as uniform and space-efficient as possible, monasteries and especially the later scribe workshops were trained in specific styles of penmanship to make their handwriting as uniform as possible. Errors in this process are cute--sometimes a scribe will doodle an illumination of a man grabbing a word and pointing down to another line where it's supposed to be or continue, for example. Each hide is a folio, which is folded over, meaning they can't just write page by page in order, so you can see markers at the bottom or top corner of each page that's usually the first word of the next page in sequence (especially common in music manuscripts). Why they didn't just number them, I can't say. Illuminations around the text were usually done after writing the actual text. After each page is finished, they bound the books together, hopefully putting their order-markers to use.
Training for scribal work was strict, as I mentioned, but you see interesting problems crop up when a scribe is copying a text but doesn't speak the language they're copying, or maybe speaks a different regional dialect. They might read another scribe's word and see something slightly different and write it down in a way that throws off the meaning. This happens a lot when a monk scribe doesn't have very good Latin, and it happens quite a lot in the Anglo-Saxon Chronicle toward the end of the 12th century; the chroniclers were keeping up the tradition of writing it in Old English, but we can tell that it was falling out of everyday use over time because the scribes get worse and worse at writing in it. Another example is Chaucer complaining to his scrivener Adam about all the mistakes he supposedly made. Thomas Hoccleve, a 14th/15th century Lancastrian scribe working in the office of the Privy Seal in England, complains about he and his coworkers getting stomach ulcers out of anxiety about how strictly they had to stick to scribal standards (he also mentions his bent back and dry eyes). An average monk working in a monastery could put out 2-3 books a year, partly because scribal work was just part of their monastic duties, but urban scribes worked on commission by the page, so they had to blaze through work as fast as they could. There was never exactly a 'standard book' to look to, in the same way carpenters don't have a 'standard chair' to look at-- they just looked at the work of their masters and what they consider fine work.
As an aside, modern rabbis use similar techniques when copying Torahs today, if you'd like a look at more recent examples.
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