Blood films are made by placing a drop of blood on one end of a slide, and using a spreader slide to disperse the blood over the slide’s length. The aim is to get a region where the cells are spaced far enough apart to be counted and differentiated.
The slide is left to air dry, after which the blood is fixed to the slide by immersing it briefly in methanol. The fixative is essential for good staining and presentation of cellular detail. After fixation, the slide is stained to distinguish the cells from each other.
Source: Wikipedia (http://en.wikipedia.org/wiki/Blood_film)