Remove any offsets. These are new stems from the plant's base. Only the monstrose form of this cactus produces these. Use a sharp knife wiped with alcohol to cut them from the base. Finesse is not necessary, but make a clean cut.
Take a cutting if no offsets are available. Remove a section of 2 to 6 inches from the plant's top. The cut quickly heals. Lay the section you removed on a flat surface indoors to dry and callus. It usually takes two weeks, but add a week in high humidity.
Fill a pot with sand. Insert the cut end of the cutting or offset into the sand until the callus is immersed in the medium. Don't plant more than 1/2 inch under the sand.
Prop up the cactus with toothpicks and place the pot in a sunny area where temperatures are at least 65 degrees Fahrenheit. Rooting takes place faster in warmer temperatures.
Water the cactus after a week has gone by. Apply only a slow drizzle of water that moistens the top 2 inches of sand lightly.