Find an Agave attenuata plant with one or more small "pups" (small Agave plants) growing from the base of the parent plant.
Cut off one of more of these small pups from the base of the parent plant with a sharp knife. Cut close to the base of the parent plant, keeping a few roots with the pup if possible (although this is not always possible).
Allow the pup(s) to lay in the shade for 48 hours, drying out slightly.
Soak the base of the slightly-dried pups in a pan of multicrop plant starter solution, available at any nursery or home improvement center. Soak overnight.
Insert the base of the pup(s) into a container containing a mix of 1/3 potting soil, 1/3 sand and 1/3 organic manure. Water well until water drips from the base of the container, and then place the container in a partially-shaded area of the garden after all possibility of frost has passed. After the initial watering, water the pups only enough to keep the soil moist but not soggy. Your Agave attenuata plant should start growing within three to four weeks.