Fill a 1-gallon container with commercial potting mix formulated for cactus. Be sure the container has a drainage hole. Use a planting tray if you want to propagate several agave plants.
Remove an offshoot--either a bulbil or a pup--from the agave plant. Bulbils grow at the base of the plant's flowering stalk and look like a miniature version of an adult plant. The bulbils are easy to pull from the stalk with your fingers. Some types of agave can be propagated by removing pups that grow at the base of the plant. The pup connects to the plant with a runner, which can be severed with a sharp knife.
Nestle the offshoot in the potting soil. Plant the offshoot just deep enough so the bulbil or pup remains upright.
Place the planting container in bright light but out of direct sunlight. Water the plant whenever the top inch of soil feels slightly dry. Water deeply, then allow the water to drain freely.
Allow the agave to root for about two months, then plant each small agave plant in an individual 4- to 6-inch pot. The agave can be planted outdoors at this time but will benefit from up to a year indoors so the roots will have ample time for development.