How do you transplant a cactus?

Very carefully. Even the smallest potted cactus can be painful to repot. When a good pair of gloves isn't enough, use a section of foam pipe insulation. Most home improvement stores carry it in a variety of widths. Cut the foam in half length-wise and use as much as needed to reach fully around the cactus stem and branches. Tape the foam pieces in place and hold gently, as you move your prickly cactus to its new home : Curtasy: about.com