Structure

(hairs still present at the top of the page)

and put through a series of processes to dispose of fat, smooth away blemishes, and refine to a smooth surface. The rougher, more yellow side of the sheet, which may be marked with little black specks in places where the hair follicles were if the animal was large, is the outer side of the skin, and the inner side is paler. When well prepared, vellum is easier to write on than paper.
The skins of a range of animals were used, depending on the size and quality required.: for a small fine manuscript, the fetus of a calf might be used, yielding a skin as fine as tissue paper. Pigskin was unsuitable, because the bristles had such large follicles the skin would be heavily pitted. Further information can be found here

Bookbinding

Large books such as these (570 x 395 mm), which may have had upto 360 pages, or 180 sheets of vellum folded in half, must have required a whole skin from an animal the size of a young steer. The sheet of vellum would have been folded in half, with the rougher, more yellow side on the outside, the folded sheets tucked inside each other like a newspaper, and then sewn together down the central fold.

When tearing the page out of the book, the wires caused these cuts in the edge of the page.

Each group of such sheets is called a gathering. The gatherings were then stacked on top of each other, and all sewn together, using the initial stitches through the center fold of the gathering as a foundation for the second stage of sewing.

Boards

To protect the codex, as this text block of pages was first called, wooden boards were attached back and front by cords which had been integrated into the sewing structure. The cords were inserted into holes in the boards and fixed with wooden pegs, and the manuscript then had an outer protective covering, which was quite often augmented by metal studs. A medium as firm and heavy as wood was necessary to keep the vellum text block in shape, because skin flexes and shifts with changes in temperature and humidity.

The "memory" of the sheet causes moist induced wrinkles when outside a hard cover

We still call the covers of a book the boards, despite the fact that for over three centuries now cardboard has been used instead.

To create huge manuscripts such as these in this way was a big commitment on the part of a monastic community. It is most unlikely that a monastery would have sufficient resources within the community to supply the skins, so the vellum would have had to be bought in. It may have been gradually accumulated over many years.