Select two coordinating colors -- one lighter than your natural color and one a bit darker unless your hair is already dark. Use a slightly lighter color for undertones in black or dark brown hair. If you crave a frosted or highlighted look, choose a super-light color for your highlights. But don't go too far with the undertones. Keep the undertone within two shades of your natural color.
Mix the dark color with an equal amount of 10-volume color developer in a color bowl. Mix the lighter color with 20-volume color developer in a separate bowl -- equal parts for this one too.
Section your hair into four equal sections. Twist it, pin it, clip it or just part it and leave it -- there's no right or wrong way as long as the hair stays where you want it to.
Start weaving using alternating colors for each foil. To weave, pick up a 1/2-inch, horizontal row of hair from the top of one of the sections. Weave the end of the comb through the section. Grasp the hair on top of the comb; let the rest fall. Place a foil under the weaved hair, holding it with your left hand. Brush color on with the right. Fold the foil to hold in place.
Continue working your way down the section, weaving alternating colors. When you finish with one section, move to the next.
Set a timer for 20 minutes. Listen for the ding, then rinse and condition your hair.
If you're a bit slow -- like if you're still weaving after 45 minutes -- you can stop the coloring process of any previously weaved sections by spraying your hair with water. The water will give you a little bit more time to get those foils in there, but only spray the parts that have processed for the full 45 minutes.
Color is not your hair's friend. Leave it on too long, and your hair will snap under the pressure. Never leave color on your hair longer than 45 minutes.