I just stumbled across this article about making your own stepping stones and I love his idea:

Cut the top 2 1/2 or 3 inches off a round five-gallon bucket and LEAVE the HANDLE attached! Use this as your mold and then after your stone sets up, use the handle to pull the mold up away from your stepping stone. I thought that was so smart!

Below is a link to the article. It's short and has pictures.

Here is a link that might be useful: Stepping Stone Instructions