There are many, many books and websites devoted to calligraphy hands and fonts, so I'll spare you from going into it here. The first thing to do is decide on the size of the font and the size of the text box. The size of the font will probably be dictated by the size of the page, how the text box sits within the page and how much you're going to write. With calfskin vellum, the size of your bank balance will also play a part! With illuminated manuscripts, the layout traditionally consists of either one or two columns per page. With a two column layout, there is a blank margin left between them.
Before writing out the chosen text on vellum or expensive paper, take time to practise, practise, practise first. I usually write out the text half a dozen times or so before plucking up the courage to tackle that pristine, pricey piece of vellum.
After having practised, the next stage is to prepare the vellum (or paper). A smooth surface is best. Papers with a textured surface are hard work and do not lend themselves to crisp, clean letters. If you're using vellum, you need to:-
Cut it to the required size. Leave large margins all the way around. With a book, the margins allow for trimming at the binding stage, and with an individual page, it allows for mounting the vellum to an under mount.
If you have the luxury of working with a whole skin, take time to inspect the surface. With vellum, the surface varies. There are hard, gnarly bits, very thin bits, holes, tough bits where veins used to be and so on. One side will be greasy, from the fat layer, and the other pitted where the hair used to be. The hair side is generally easier to work with. Cut your piece from the best part of the skin. Having the spine running horizontally across a large page gives greater stability.
Transfer the page layout to the page. Either measure carefully or use a template drawn out on a piece of thin paper. Using a sharp point (whatever is available), prick the vellum at the corners of the text box.
Then prick either end of each margin and text line.
A section of text from The Gospel of Thomas (illuminated manuscript by Diane George). The blind rulings of the text lines and margins can just be seen (I've enhanced them to make them easier to see). The sense of the text is clarified by capitals and leaving the remainder of the line blank in place of punctuation.
Using a blunt metal point or bone folder, rule the lines. Rule from point to point. Do not extend the lines outside of the text box (unless you're intending them to guide margin decoration). Apply quite firm pressure. Vellum is quite tough.
Having ruled all of the lines: in the centre of the vellum, add a small pile of pumice powder.
Working out from the centre to the edges, work the pumice powder over the surface of the vellum. Do not work in from the edges, as this may lead to creasing of the skin. Always work from the middle, out. The pumice will de-grease the surface.
With a large, soft brush, remove the excess pumice.
Pounce powdered gum sandarac over the area to be written on. Brush off the excess carefully.
From now on, DO NOT TOUCH THE SURFACE. You will transfer grease from your hands to your carefully prepared vellum. Protect the areas that you are not writing on with silk or clean paper. A pad of silk under your hand is a good idea.
Write the text along the blind ruled lines. Keep within the text box.
The double blind ruled lines can be seen here, guiding the text.