how do you "illuminate"?

Macclesfield Psalter, a lavishly illuminated manuscript from Great Britain, written in Latin, circa 1330

the text is written first on sheets of parchment or vellum, i.e., animal hides specially prepared for writing, cut down to the appropriate size. 

after the general layout of the page is planned (e.g., initial capital, borders), the page is lightly ruled with a pointed stick, and you (the scribe) go to work with an ink-pot and either sharpened quill feather or reed pen.

here usage depends on local customs and tastes. 

the Rustica Roman (from the early middle ages) gradually gave way to scripts such as Uncial and half-Uncial, especially in the British Isles, where distinctive scripts such as insular majuscule and insular minuscule developed.

(above you see stocky, richly textured black letter first seen around the 13th century and particularly popular in the later Middle Ages)

illumination takes pre-production to outline the work. you don't want your lettering all crowded into a format dominated by huge ornamented capitals that descended from uncial forms or by illustrations, so you plan for blank spaces for the decoration. see that the scribe and the illuminator are separate labors.

here is an approximate order of tasks:

1- silver-point drawing of the design is executed
2- burnished gold dots are applied
3- application of modulating colors
4- addition to the outlining of marginal figures
5- the penning of a rinceaux (or foliage) appearing on the border of a page
6- finally, the marginal figures are painted.

some examples of rinceaux:

Buffalo, NY (early 20th century)

Albi Cathedral (1280 CE)

Apse, San Clemente al Laterano (500 CE)

Mosaic, Pella, Greece (400 BC)