We used old 9 x 13 cake pans. Other people on family crafts at about.com used foil pans, pizza boxes, flower pot bottoms, the possibilities are endless. We used garden finds to decorate our stones, but you can decorate with glass stones, beads, paint, hand prints, whatever you would like.
We used a 60 lb bag of quick-kret (it only cost a couple of dollars). Wearing latex gloves, we poured the bag into a 5 gallon bucket to make it more manageable. Then we scooped out smaller amounts and mixed it in a smaller bucket. A 60 lb bag made 4 - 9 x 13 stepping stones. We added water and stirred to make a concrete that was like a thick cake batter. We lined the pans with plastic wrap, filled them with the concrete and then gently tapped them on the ground to get the air bubbles out. Using paper towels we blotted some of the excess water. Then we collected leaves and flowers and pressed them into the concrete. We let them sit for a few hours and then pulled the leaves and flowers out and let the stone finish drying. After 3 days (DO NOT REMOVE IT SOONER) we took the stone out of the pan and let it finish drying for another week before putting it in the ground.
Check out family crafts at about.com to see other stepping stone ideas, supply lists, tips and gallery.