Summarize this article:

Make sure that you put the gloves on before you begin mixing any bleach. Bleach can be very damaging to skin and fabrics, so the towel will protect you and your clothes from any bleach that drips from your hair  Many bleach kits will come with the gloves for you to use, but your own plastic or latex gloves are also fine. A hairdressing cape will also work well instead of an old towel if you have access to one. The ratio of the bleach powder to the developer will be specified on the bleach kit box. Make sure that you use a non-metallic bowl. Place an old towel beneath the bowl to avoid the bleach damaging the surface. Remove this section from the plastic clip, and keep the rest of your hair clipped back. This section should be about 1 inch (2.5 cm) thick if you have thin hair, or thinner if you have thick hair. Bleaching your hair in small sections will make sure that the bleach is applied thoroughly and evenly throughout. Make sure that the bleach covers the full length of your hair, but leave the roots uncovered since the bleach may irritate your scalp. Brush the bleach through in the same direction that your hair grows, starting at the top and ending at the tip. Work as quickly and carefully as possible, so that the first section doesn’t lighten too much before you have put the bleach into the last section. Apply bleach to this side too, brushing it through in exactly the same way. This will ensure that you don’t accidentally miss a spot. You can group this section together, and clip it back with a plastic clip again to keep it out of the way while you bleach the rest of your hair. Repeat the bleaching process through the rest of the first quarter, the other back quarter, and the two front quarters.  Remember to leave the roots unbleached for now. By the time you have finished the final front quarter, the first back quarter will be starting to lighten. After you have bleached all of the sections of your hair, you can also do the same to the 1 cm (0.4 in) of root hair. Bleach can make your scalp feel itchy or slightly irritated, so start out with a small, unnoticeable part of your roots first to see how it feels. You can leave your roots unbleached if the feeling is too unpleasant.

Summary:
Put on disposable gloves and drape an old towel over your shoulders. Mix together the bleach powder and the developer with the tint brush. Take a thin section of hair from one of the back quarters. Apply the bleach to this section using the tint brush. Flip the first section over to show the side that hasn’t been bleached. Continue applying bleach to thin sections until each quarter is complete. Apply bleach to your roots if you want them bleached too.