Part your hair across the back of your head, from ear to ear, using a comb. Make this part even because it will delineate different areas of color. Ask a friend to do this part for you, if you can.
Secure the top layer of your hair using hair clips. If your hair is long, braid it or put it in a bun to keep it out of the way. Use mirrors or enlist the help of a friend to make sure no top-section stragglers are left with the bottom layer of hair.
Wrap the top layer of your hair in plastic wrap to protect it from the dye. Place the bottom edge of the plastic right at the part, and wrap both ends of the piece around your head so that you can twist them to secure the wrap in front. Use clips to secure the plastic into your hair if necessary. Take your time with this step; if you get dye on the top layer of your hair, the result will look raggedy and unprofessional.
Dye the bottom layer of hair using a hair dye kit, following the instructions on the box. Wear plastic gloves that come with the kit while mixing and applying the color. Smooth the dye onto the bottom layer of hair, avoiding the plastic-covered top layer. Allow the dye to develop for the amount of time indicated in the directions.
Rinse your bottom layer out while the top layer is still protected to avoid getting dye into the un-dyed hair. Once the water runs clear and any excess dye is gone, undo the top layer. Dry your hair and enjoy your new, two-toned look!