This is a more complex question than it might originally seem. Whole books could be written on the subject – and have been! (For which, see below.) I can only cover so much at once, so the following comes with a few caveats:

1. My knowledge of book history is chiefly limited to books produced in Britain, mostly in the later Middle Ages. That isn’t to say that none of this applies elsewhere! As far as I know, it is broadly true that the structure of the codex (the bound “book” as we now think of it) was more homogenous across Western Europe and indeed across the span of the Middle Ages than many of the subjects we tend to discuss here. But I confess this comes from more than a little ignorance and assumption on my part, and I would be very pleased to be corrected.

2. I limit my discussion to the codex, but I do this only because you asked specifically about books; I would be happy to talk about other forms at more length if anyone’s interested. Medieval “information technology” was by no means limited to the codex – people stored written information in a very rich variety of ways. Rolls such as BodL Lat. misc. e. 22 are the obvious example, and they remained useful for all sorts of things after the invention of the book. But people also made folded formats such as BodL Ashmole 6 and BodL Rawl. D. 939, both kinds of almanac. People drafted on wax tablets which occasionally survive, such as these Late Middle English examples from Swinegate, York. Text was constantly being copied outside of the boards of the codex, and I necessarily omit discussion of the way this affected copying practices.

3. There is a lot of nuance to be drawn out in terms of how book production was affected by the texts recorded in the books themselves. Copying a bible is not like copying a long poem; is not like copying a prose astrological treatise; is not like copying a composite miscellany. This can be quite the rabbit hole. As with forms, I can delve further into this if it’s of interest to anyone.

All that said, it’s probably best to begin with the basics of how the medieval codex was made. Let’s say you want to make one – how, exactly? Take skins of parchment (and, later, sheets of rag paper), and fold them to the desired size, then cut the edges to produce a single bifolium, a sheet of two leaves (folia), or four pages. Fold them in the middle, and lay them together in gatherings (typically eight, ten, or twelve leaves – sometimes far more, or even fewer), and sew them together along the fold, or gutter, to form a little booklet, or quire. Confusingly, book history actually uses booklet to mean something else – more on this below. Eventually (usually after the text is copied!) the quires are sewn and/or glued into the boards, and the codex is born (although not necessarily finished, for which see below). Book producers sometimes cancel leaves at various points in the process if a quire is too large for the text, or introduce singletons if it isn’t large enough – very commonly, blank pages at quire ends in manuscripts testify to an overabundance of material.

But what of this business of copying? There is rather more to this than just putting the text on the page! The majority of mid- to high quality medieval books were pricked and ruled; using an awl or knife and a ruler, the producer/scribe measures evenly spaced holes down the margins (if in a hurry, this can be done with folded sheets at double speed!), then rules between them to create lines upon which to write. Drypoint is standard, and leaves only faintly visible lines, but grey plummet is an option. Ink becomes more fashionable in the later Middle Ages – some very fancy French and English manuscripts of the fifteenth century are ruled in lilac! You can just about make it out in the BodL Bodley 686 Canterbury Tales, although the picture distorts the colour a little. Once the quires are ruled (or not, in lower quality manuscripts) it’s time to get writing.

As I alluded above, copying seems mostly to have been done in loose quires, before binding. These could be laid individually flat, or tacketed by sewing them loosely together. The task of keeping these quires in order during copying actually had some fairly straightforward solutions. Little numeric page tallies (usually Roman, increasingly Arabic from around the turn of the fifteenth century), and quire signatures (alphabetic tallies by quire) kept things in order. These survive on untrimmed pages, usually in the bottom corner of the recto pages in the first half of a quire. Here’s a good example – you can just make out “d ii” that indicates this page was on the second bifolium of the fourth quire. Keeping order between the quires themselves had a neat solution; catchwords such as this (“vr lord he bad, our lord he bade”) on the verso of the last quire would match the first words on the next quire. Sometimes these would later be decorated, to prettify their necessary inclusion in the book. Scrolls are overwhelmingly popular, but some artists could be very creative indeed.

A scribe’s work was not necessarily done when the text was copied and the quires signed, either. Correction of error is extremely common; scribes often correct their own work, but other hands do too – sometimes, in collaborative cases, scribes correct each other. In the Beowulf manuscript, famously, one of the scribes zealously fixes the work of another. Many corrections are made as we make them now – with overwriting, insertion with caret marks, or strikethroughs and marginal additions. But scribes and non-scribe correctors also underdot errors, literally putting dotted lines beneath them not unlike modern spellcheck squiggles. A scribe with a knife handy can also make erasures by gently scraping away the top surface of the parchment (this is trickier, but not impossible, with rag paper). In commercial manuscripts, correction could be a selling point – some books are marked “corr.” or similar to indicate that they have been professionally checked before purchase.

There are other jobs too. Some scribes would have been served as illuminators, particular in books where decoration is restricted to starkly decorated catchwords or initials – the black strapwork initials often associated with Ricardus Franciscus could be this kind of extra-curricular work, as could the red and blue paraph marks which were popular from the thirteenth century onward. When the scribe is not the illuminator, they may add little guide letters to make sure the illuminator wrote the write decorated initial in the space (this didn’t always stop them getting it wrong!). Some books call for running titles, intermediary titles and colophons, or diagrams and all matter of other apparatus. It is no surprise that scribal colophons at the ends of books conventionally bemoan wrist-pain, or beg for drink!

Stepping back a bit, the last major aspect of book production I ought to note, particularly in regard to keeping track of folia and quires, is that some copying of large works – particularly in the later Middle Ages – was done collaboratively. The most well-known studies in English books have related to the production of poetic manuscripts in London, and have suggested that groups of scribes (often people whose main work was not actually as book producers, but who made books as freelance work) seem to have had access to texts like The Canterbury Tales in piecemeal exemplars, and passed those quires round to copy their own quires which were later assembled into whole books. This is where the technical sense of booklet comes into play – discrete units of arrangement within single manuscripts, brought about through deliberate conceptual grouping or the more contingent groupings produced by travelling exemplars and scribes. Even when single scribes are at work this can come into play – much ink has been spilt on the subject of how “The Exeter Book”, an Old English poetic compilation, was arranged, and to what degree its internal booklets reflect the order and availability of the single monastic scribe’s exemplar/s. There is a lot more to be said here; almost every book is different in terms of how its composition of texts interacted with the physical condition of the scribe/s access to an exemplar. Perfect facsimile copies seem to have been the exception, not the rule (although a tiny number of identical copies of some books do indeed survive).

As I’ve tried to stress, this is a subject of breadth and depth! See my reply for the sources for much of what I’ve said, which would serve as a decent starting point for any study of the medieval history of the book (at least in Britain).