Remove the terminal 6 inches from one of the orchid cactus's healthiest, most prolific stems.
Place the cutting on a paper towel and allow it to dry out in a cool shady spot for one week.
Fill a small plastic planting container to within 1/2 inch of its lip with a cacti or succulent potting mix. To make your own, mix 3 parts potting soil with 1 part pumice, perlite or bark chips.
Plant the cutting growing-tip-down (with the cut end sticking up) 1 to 2 inches deep so that no more than two leaf "serrations" are submerged. If the cutting will not stand up on its own, tie it loosely to a houseplant stake (available at most home and garden centers).
Place the pot in a spot where it will receive full (six to eight hours daily), indirect sunlight.
Mist the cutting once daily (mist only, do not water) for three weeks. Then water the potting mix with a spray bottle to just moisten it to its depth. Resume misting for three weeks and then water again. After the second watering, begin watering the plant weekly or whenever the top 1/2 inch of soil dries out.
Check the plant to see if it has rooted by pulling up on it gently. If it resists, it has roots.
Plant the cactus orchid out in its permanent location one month after you check it for roots.