Your picture didn't make it, but it sounds like you have a nice substrate on which you can use thinset to adhere more tiles.

If you are going to glue your mosaic then grout, it would be easier (and maybe less expensive) to just buy ready made stepping stones to work on. If you don't want to grout at all and make your stones, then you would want to lay out your design on some clear contact paper and put that in the bottom of the form and then pour your concrete (with the contact paper on the bottom, sticky side up). Depending on the size of the stepping stone, you may want to put some wire in the concrete for reinforcement. If you don't want little pebbles, then use the concrete w/out pebbles. I've done it both ways (with glass) and each process looks good. I'll send some pics when I get home.

Sounds like U got some good advise. I think I would do like Slow says and use thinset to put on mmore tiles then grout...Let us know what U do and add some pics please. We all love seeing those pics

halley, as promised, here are the pics of my stepping stone. All except the sunflower were made using the indirect method (i.e., putting the glass on contact paper, placing the contact paper face up in the mold and then pouring the concrete on top).

The sunflower was made using a pre-made stepping stone, gluing the glass onto the top and grouting the top and sides.

One other thing I would like to point out is if you are using a material that is slippery (glass or glazed tile) be sure to leave enough concrete or grout exposed to prevent slipping when stepping on the stones when they are wet.

Good luck and show us picture of your finished stones!