I used the plastic store bought molds. I cut a piece of contact paper to fit the bottom of the mold. Use the clear contact paper. Draw your design with permanent marker. Remove the paper backing to reveal the sticky side. Put it in the bottom of the mold with the sticky side up,
Now use your glass, stones, etc. to fill in your design outline. Mix your concrete and slowly spoon over your design and gradually fill to the top,
Let it cure 24 hrs or so and then remove from the mold. Turn over so the contact paper is on top, Slowly pull off the contact paper from the concrete and like Dave, clean up any concrete film that may have seeped onto the design. Finish curing in water as per Dave's instruction.
I have lots of scrape stained glass and plan to make more mosaic stones when I have some spare time.
