Spring is the best time of year to propagate these leaves.

Allow each of them to develop a callus where they broke away from from the parent plant. That takes about 7 days but if necessary allow more time until the wound has a layer of protection. For each leaf, fill a small pot with some lightly damp potting mix (either a cactus mix or a regular potting mix with healthy amounts of sand or perlite added). Then lie each leaf on its side on the surface of the potting mix. The area where the leaf was detached from the parent plant is where new roots and a new stem will emerge so that area should be just in contact with the soil surface. Gently settle the leaves into the soil surface so they don't roll around and the detachment spot stays in contact with the soil mixture.

Keep them at a steady warmth of about 70 degrees and in indirect light.  It is possible to purchase a propagation mat that stays at a fixed temperature if you don't have a spot of steady warmth. Keep the soil surface just damp for several weeks. You should see new roots and stems form in about a month. 

Once the little plants are clearly rooted and growing, give them a few weeks before you remove them from the propagation mat and into more sunlight, treating them as adult plants.

Courtesy of NYBG Plant Information Service