Purchase a small selection of Prismacolor markers to get started. Since they are so expensive, it's best to start out with a set of grays and a few neutral tones. You can add more colors as you hone your skills.
Lightly sketch a preliminary drawing on your paper with a pencil.
Ink your drawing with the fine end of a black marker. Vary the width of your line in strategic places by applying light pressure as you draw.
Allow the black ink to dry, then erase your pencil sketch.
Block in large, flat areas of color with the chisel end of your marker, drawing with a circular motion. Let these big areas of color dry before proceeding.
Layer over your first coat with the same colored markers in the areas where you want shadow. Use a white colored pencil to add highlights.
Use the fine side of your marker to add details. Use this end sparingly, however, as it wears out more easily than the other end.