The succulent in the center already had a good root system so that one is actually buried in the potting mix. As for the others, I simply arranged the succulent leaves as I wanted on top of the soil. As the leaves broke down, they gave way to new plants which will be rooted in the potting mix.
Even though I used good quality potting mix and plan to fertilize when required, it will take a few months for these to grow in. They have a nice, sunny perch near a window and will be most delightful to look at. Can't hardly wait!