Sometimes you need to create a mosaic surface that is perfectly flat, such as for stepping stones that people and animals will be walking on, in order to avoid sharp edges and thus stubbed toes. In this case, you should use the "indirect method." In this method, you create your design upside down so that the top surface, when you flip it over, is flat. It takes a few more steps than the "direct" method, in which you simply arrange your tiles or glass on top of a stepping stone base, such as a patio stone.
For the indirect method, you will need a mold, some brown kraft paper, a release agent, clean sand, cement mix, a piece of chicken wire, and sanded grout.
You can use a readymade mold such as those sold at craft stores, or you can make your own: Cut pieces of lumber, such as 2x2s or 2x4s (but any size will do), into equal lengths, and then use wood screws to form them into a square. Set the wood form onto a scrap piece of plywood. Cut a piece of craft paper to the size of the inside of the form and lay it inside the mold. Grease the inside of the form and the paper "floor" with a release agent, such as cooking spray or petroleum jelly (anything greasy will work).
Now lay out your mosaic design inside the mold, upside down. This is easy to do if you are using glass tiles, but if you're making a ceramic mosaic, you won't be able to see the colors when they are turned over; in that case, lay out your design right side up to the dimensions of the paper square on a piece of heavy cardboard, then grease the paper and lay it on top of the design. Put another piece of cardboard on the top and carefully flip the design over so your tile design is now upside down. Slip it into the mold, removing the cardboard. Readjust any pieces that might have slipped.
Now sprinkle some clean sand over the mold all around the pieces. Keep it no more than one-half the depth of the tiles. This will leave a space for grout after the cement has dried. I use a small brush to wipe sand off the tiles and keep it in the cracks.
Mix up some concrete according to package directions; the ready-mix kind is fine. Pour it into the mold about halfway to the top, then cut a piece of chicken wire to the size of the mold and lay it on top of the concrete. Continue filling with the rest of the concrete to the top of the mold. The chicken wire provides additional strength to keep the concrete from cracking. Let it dry for a few days; I follow the concrete manufacturer's dry time recommendations and add a day just for good measure.
Turn the mold over and gently tap it to release the concrete stepping stone. Do this outside, because the sand you sprinkled over the top will start to leak out. Turn the stone right side up and use a hose to thoroughly wet the paper; within a few minutes it will peel off in large chunks. Clean out any remaining sand with the hose.
Mix up sanded grout according to package directions. Push it into the crevices around the tile pieces with a grout float, then scrape off most of the excess. Let it sit for about an hour, then dampen and wring out a grout sponge and wipe off the rest of the excess grout, taking care not to remove grout in the crevices. After another hour or so, clean off the top of the stepping stone with clean rags or a clean sponge. The next day, spray with a grout sealer to retard mold and mildew, and your stepping stone is ready to install in your garden.