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Don't bleach hair that is freshly washed. The bleaching process will dry your hair out significantly, potentially causing damage and breakage. Your hair's natural oils will prevent this. A freshly-washed scalp is also much more sensitive and will itch more during bleaching. Your bleach kit will have two main components: dry powdered bleach and a hydrogen peroxide solution. Mix the two according to the ratios or instructions on your dye kit, using a tint brush or plastic spoon.  Follow the instructions on your kit if they are different from what is described here. Developers come in different strengths rated by numbered "volumes." If your hair is already fairly light, use volume 10. Volume 20 is for darker blondes, volume 30 for light browns, and volume 40 for darker browns and black.  Higher volumes can be more damaging to your hair, but they can also lighten it faster. If you've never bleached your hair to platinum blonde before, you will need to know how long it takes your hair to lighten. Select a small amount of hair close to the roots at an inconspicuous place and section it off. Brush on some bleach using a tiny brush. Let the bleach set and check the strand every 5 to 10 minutes. If your hair is not light enough after an hour, you will need to do multiple rounds of bleaching. Rinse the bleach away between rounds and reapply it. To minimize damage, do not leave bleach in your hair for over an hour. Use a tint brush to apply your bleach mixture evenly to your hair. Work in sections, brushing in the direction your hair grows.  Start from the very back of your head and work your way forward, leaving the sections of hair that frame your face for the end. If your hair is thick, flip up each section to expose the next layer of hair underneath and apply there, too. Leave about a one-inch section around your roots to do last, after the face-framing strands. The heat from your scalp will cause your roots to bleach a little faster than your normal hair. Try to go as quickly as possible to ensure an even color. Refer back to the results of your strand test to determine how long you should leave the bleach in your hair. Cover your hair up with either a shower cap or plastic wrap while you wait. Keep in mind that a little itchiness on your scalp is normal. Use warm water without shampoo. Be very careful to not get any bleach in your eyes as you rinse. If possible, ask someone to rinse your hair for you in a sink while you look up as if you were in a salon. If you aren't immediately moving on to the next step, allow your hair to air dry. Blow drying can further strip your hair of moisture, potentially exacerbating any damage the bleach has caused.
Wait a day or two after your last shampoo. Mix bleach powder with developer. Do a strand test. Apply the bleach. Let the bleach sit. Rinse out the bleach.