1. Immerse a concrete stone in water to wet it thoroughly. Prepare thin-set mortar in bucket according to package directions (consistency should be similar to slightly runny peanut butter). Using notched trowel, spread a 1/4- to 1/2-inch layer of mortar onto part of stone.
2. Arrange pieces as desired, pressing them lightly into the mortar and continuing across the stone. Add extra mortar under the pieces, as needed, to achieve a fairly even surface. Leave crevices between all pieces for grout. Clean excess mortar from the surface and let the stone dry overnight.
3. Mix grout according to package directions. Scoop a large amount onto the stone, using a spatula or trowel to spread it out and push it into gaps. Add as needed, removing excess as you go. Smooth grout on the sides of the stone as well. (Note: You may use mortar in place of grout.)
4. Using a wet sponge, clean excess grout from the sides of the stone. Wipe down the top, going over the surface in both directions and rinsing the sponge often. Repeat, taking care not to remove too much grout from the gaps. Let the stone dry 24 to 48 hours, then buff it with a soft cloth.