Lay a tarp or large plastic sheet out on a work area that is protected from moisture and temperature extremes.
Put on your rubber gloves and disposable breathing mask. Mix your concrete in a wheelbarrow or large bucket, following package directions. Use a hoe or other strong mixing tool when handling this big mixing job.
Prepare your stepping stone molds by spraying them with a coat of non-stick spray. Non-stick cooking spray or an oil lubricant spray will work well.
Spoon the prepared concrete into your molds, then gently tap the side of the molds to release air pockets and smooth the surface. Give the concrete a few minutes to set up, and then arrange your leaves on the top in a pleasing pattern. Make sure the veins on the bottom of the leaves are resting on the concrete. Gently press the leaves into the concrete, but don't allow the concrete to flow over the sides of the leaves. If you like, add other decorations to your leaf stepping stone, such as a pebble border.
If you prefer, you can put your leaves and decorations in the bottom of the mold, vein side up, and then spoon the concrete over them. Tap the sides of the mold and smooth the surface.
Allow your new garden stepping stones to dry for three or four days.
Turn the molds over and gently push or tap out each stepping stone. Allow the stepping stones to rest seven more days before you put them outside.
Place your stepping stones in the garden. Bury them so that the tops are flush with the top of the soil.