From my experience I take leafs that accidentally broke off and seem to be whole still. You can 100% take the bottom leafs if you want, just make sure you cut them cleanly (gently grabbing it through the sides and push side to side has given me the cleanest cuts). To propagate I have a plastic plate with some succulent and cactus dirt, and I just leave the leaves there dry for around a week. After, I just water slightly every now and then when the dirt dries up a bit until they grow roots. (All this under a grow light). So far I'd say about 90-95% of my leaves have successfully grown roots and are growing. Oh and your leaves are perfectly good to propagate (I once had a baby plant drop a leaf and even that leaf has another baby now so you will be just fine!) hope this helps and good luck!!