Crappy clothes . Don't dye your hair without having as much of your skin covered as possible. An old towel , for wiping gunk off your forehead, neck, and ears. A large plastic bag and hair clip . A timer .
Different products have very different instructions.
For example, some require your hair to be dry, and others wet. You don't want to be fumbling around with the directions when you've already dumped the bottle of goop onto your head. So mix your bottle , following the instructions exactly as they are written. After the bottle is thoroughly mixed, work the dye from the roots out , making sure your hair is eventually completely and evenly saturated. You might want to use a comb to help you spread the gunk through your hair and to smooth out the clumps.