This project is one of my favorites. Go to your local tile store and ask to see their damaged tile bin. With a little sweet talking, you could practically have them give you the tiles you want. You can use a set of pliers to break the tiles into the size pieces you want, then lay the pieces out in your mosaic on a sheet of paper.
When you pour your concrete for your stepping stone into the mold, smooth it out real pretty, and let it set for about 30 minutes or so. Then, insert your mosaic tiles into the concrete surface, working quickly before the concrete gets too hard. Make sure the surface ends up nice and smooth.
These are just WONDERFUL!
PS. The Duck Pond in my new living room is finished, and the chickens on the roof are beginning to produce eggs. The few I tried out as homing chickens just ended up hanging around with the pigeons in Central Park.