The scribe often left instructions for the illuminator. Within or near the space left out for splendid initials, often a tiny letter indicated what letter should be included there. Occasionally, as in 13th century Cistercian manuscripts that used a single color for initials, a spot of paint was included to indicate the color of the initial. The names of colors were sometimes inscribed on the margin or within the design itself.

The illuminator began by laying out the design in leadpoint or graphite. When the underdrawing was completed, it was reinforced with ink.

Pigments would be mixed with some sort of binding medium that kept the pigment particles together. Until the fourteenth century the most widely used binding medium was glair. Glair was an optimal medium for miniature painting, free-flowing and easily applicable, but was fairly difficult to prepare well. Besides, it reduced the natural saturation of colors, and was therefore sometimes varnished with honey after drying. After the fourteenth century glair was replaced with gum arabic (the gum of a domestic tree). Its advantages included the fact that it could be more thinly applied, thus the resulting colors were more transparent and saturated. The two types of binding media were sometimes used together or occasionally mixed with further types of binding media such as egg yolk, sugar, or ear wax.

The technique, which both the glair and gum arabic mediums encouraged, was similar to tempera painting, that is, slow-paced, careful work with tiny, meticulous brushstrokes, creating clearly defined forms and homogenous areas of color. The painting of the illustrations started with application of the basic colors in numerous layers of washes, and with outlining the original lines of the sketch. Then darker tones were added, followed by the addition of the highlighting to add a sense of depth. The use of very small brushes and the easily controllable, free-flowing medium was a prerequisite for the execution of the minute pictures that were usually rendered in great detail. When the book was still in separate leaves, the artist could work on a number of pages at the same time, and mix colors for use on more than one page.