Measure the width of the mold using a tape measure. Add 1 inch to the width and use the measurement to paint parallel lines on the ground with landscaping paint along the pathway upon which you wish to set the stones.
Use a spade to remove an inch of topsoil between the painted lines. Tamp down the ground in the excavated pathway with a tamper to compact it and level the surface.
Mix the quick-setting concrete according to the concrete manufacturer's instructions in a large bucket or wheelbarrow. Only mix as much as you can use in 20 minutes to avoid the concrete's setting as you work. Pour the concrete mix into the bucket, and then mix in the water with an electric drill with a paddle bit attachment until the mixture has the same consistency as peanut butter.
Place the mold onto the pathway centered between the two edges.
Fill the mold with concrete using a trowel to pack it to the mold brim. Smooth the concrete on the top of the mold with the trowel. Then remove the mold leaving the molded concrete stepping stone behind.
Submerge the trowel in a bucket of water. Use the wet trowel to smooth the edges of the stone.
Set the mold onto the pathway adjacent to the placed stone and repeat the process until you've created the entire path. Wait between five and seven days for the concrete to cure.
Cover the pathway with masonry sand. Use a broom to sweep the sand into the joints between the stones until the sand in the joints is level with the path top. Sweep away excess sand from the steeping stones, then wet the sand slightly using a water hose to set the material.