TAKE A STRAND TEST: Snip a small piece of hair from your underlayer and color this first. If only I had, I could have spared myself the brass debacle!
HALF TIME FOR PROCESSED HAIR: If your locks are already color-treated (like mine), they will absorb new color faster and will need only about half the dyeing time recommended on the box.
HIT THE BLEACH: Before you buy, check out the highlighting kit's volume number (you'll have to open boxes-be sneaky). On the side of most bottles, there is a number before the letter V for "volume," indicating the potency of the bleach. Select the highest available (30 or 40V); anything lower won't lift enough and could leave you orange.
LOOK, NO HANDS: I skipped the protective application gloves but (youch!) don't recommend it. Think back to chemistry class: Bleach burns.
DAMAGE CONTROL: If you do end up brassy, apply an ashy version of the color you want, then leave on for five to 10 minutes to temper the orange.