To take a leaf for propagation, just gently twist the leaf off the stem. Make sure it's a clean pull, leaving nothing on the stem. In fact, it's fine to pull of a little of the stem, too.
If you're working with leaves, set them on top of the soil, making sure their ends don't actually touch the soil at all, and water them each time the soil dries out. Use a spray bottle to get the top of the soil wet.