Ask Mr. Smarty Plants

Very carefully. Seriously, if you are going to mess with prickly pears, you must not only avoid the spines, which are actually modified leaves, but also watch out for the glochids, which are almost invisible little hair-like structures on the cactus pad, and they can seriously damage you if you get tangled up in them. They can even affect your lungs! Most of the propagation information we found on Opuntia involved allowing them to vegetatively root, which they will happily do on their own, or you can cut the pad, allow it to callus, and then put the callus-side down into potting soil. Apparently, it's a lot easier to propagate an opuntia that way than by seeds. But, if you're determined, the only information we found about obtaining the seeds (aside from ordering a packet from Amazon) was to cut the fruit. Wait until the fruit is red, so you know it's ripe, then cut it in half, scoop out the seeds, rinse them and let them dry. The best we can tell, the rate of germination of these seeds is pretty low. They apparently do much better if they have been eaten by a small animal, like a rabbit, and then redeposited on the ground with some fresh fertilizer, no charge.

Well, you did ask.

Opuntia engelmannii

Opuntia macrocentra

Opuntia macrorhiza

Opuntia ellisiana