Fill an under-bed storage box with sand. Dig the sand away until you have a leaf-shaped hole.
Prepare ready-mix concrete in a wheelbarrow or large, plastic storage bin. Add the coloring agent according to package directions and mix until color is evenly distributed throughout the concrete mix.
Fill the hole in the sand with concrete mix. "Rod" the concrete by poking it in several places with a chopstick or dowel to release any air bubbles. Air bubbles in the concrete will create weak points, according to Materials Science and Technology Workshop director Jennifer Lewis. Allow concrete to dry for a few minutes.
Press the ribbed side of your chosen leaves into the concrete is still wet. Remove the leaves before allowing the concrete to finish setting.
Remove your leaf-shaped, sand-cast stepping stone from the sand. Pour the sand into the hole where you wish to place your paving stone to make it easier to level.
Mix a fresh batch of concrete in a wheelbarrow or large plastic storage bin.
Add coloring agent if desired. Pour concrete into ready-made round or square mold frames.
Smooth the concrete with a trowel to ensure that the surface is flat. Rod the concrete as in Step 3, Section 1.
Position aquarium gems, pebbles, shells, seed pods, mosaic tiles or marbles on a sheet of white paper until you are pleased with the arrangement.
Lay each item on the molded concrete in the same position as it occupied on the white paper. Press each item into the drying concrete at least halfway.
Apply several coats of clear acrylic sealant to your stepping stone mosaics after they have dried for three to five days. Allow acrylic to dry completely between each coat.
Place your mosaic stones where you wish.