Saw the board into four 18-inch sections. Set the board sections so the 18-inch by 2-inch sides are against a flat surface. It should look like a bottomless square box when looking down at it.
Fasten the boards together at the corners using the drill or cordless screwdriver. This is the frame that will hold the cement for the stepping stone.
Line the frame with plastic sheeting (or a garbage bag). Avoid leaving wrinkles on the flat surface because they will show on the finished stepping stone. Place the leaves vein side up in the frame--on top of the plastic.
Mix the cement in the bucket or wheelbarrow. Mix a quart and a half of water with the 40-pound bag of Quikrete as per the directions on the bag--but confirm quantities with directions for other brands.
Put cement on top of the leaf or leaves carefully using the trowel or small shovel. When the leaves are covered, pour the rest of the cement mix into the frame.
Allow the stepping stone harden a minimum of 24 hours, but it can take as long as four days. According to Complete Home, 24 hours should be enough for the concrete to harden, but not to full strength.
Try to lift the frame off of the stepping stone. It might lift off but if it is too snug, use the drill or cordless screwdriver to remove the screws in the frame.
Peel the plastic sheeting away from the stone. The leaf will wash away with water--or it can be left in place to decompose in the cement.