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This is important as the oils on your hair will help protect it against damage. When you do wash your hair 24 to 48 hours prior, make sure that you only use shampoo. Conditioner will prevent the dye from adhering. Put on a shirt that you won't mind staining, then wrap an old towel or a plastic cape around your shoulders. Coat your hairline, the tips of your ears, and the back of your neck with petroleum jelly. Finally, pull on a pair of plastic gloves.  Work in an area that is easy to clean, such as a bathroom or kitchen. If you are worried about staining the counters or floors, cover them with newspaper, paper bags, or plastic bags. It is easier to dye your hair in layers .5–1 in (1.3–2.5 cm) thick rather than in sections. Part your hair across the back of your head at about ear-level. Pull everything above the part into a bun.  Secure the bun with a claw-clip. It'll be the easier to remove and replace. If you have very thick hair, you may want to part it even lower so that you are working with thinner sections. If your hair is too short to pull into a bun, use clips. If you have chin-length or shorter hair, you may not have to do this step at all. Some dyes come in a kit that already include the dye and developer. For other dyes, you must purchase the developer separately. Follow the instructions that came with the dye to find out how you should mix them.  You can mix the dye in the squeeze bottles that came in the kit, or you can mix it in a non-metal bowl.  If you are lightening your hair, add 1 to 3 packets of toner. The more packets you use, the ashier the final color will be.  If you can't find packets of toner, don't worry; you can wash your hair with purple shampoo afterwards to remove brassy hints. Take a thin strand of hair from an inconspicuous area, such as your nape, and apply the dye to it. Cover the strand with plastic wrap, and let the dye sit for the time recommended on the bottle. Rinse the dye out with cool water, then let it dry.  You don't have to use conditioner for this since this is just a strand test. Although not absolutely necessary, strand tests are highly recommended because the color may come out differently from what you expect. If the dye did not turn out the way you wanted it, you’ll have to buy a different color.

Summary:
Start with dry, brushed hair that hasn't been washed for 24 to 48 hours. Protect your clothing and skin against stains. Pin your hair up, leaving only the bottom layer loose. Prepare your dye according to the instructions on the package. Perform a strand test to gauge the color.