Mix the hair dye with the developer. Or, mix the bleach with the developer if you are using bleach. Start off with one strand, making sure it turns out to your desired color. Be sure to coat EVERY strand of hair with the dye, making sure your hair extensions are laid out on a towel. After that, wash hair with cold water. Do not rub the hair together vigorously. Run the water on them and use your hands to detangle. Make sure that all of the dye is completely out and the water is running clear.