You don't have to build a foundation of sand and gravel when you lay down cut stepping stones. If your stones have some heft, you can just remove sod and soil to the appropriate depth and call it a day.
1. Lay out the path. Position your stones and outline them with your spade.
2. Remove sod. Lay the stone aside, and surgically remove soil with horizontal strokes.
3. Install the stones. Plop the stone into the jigsaw-puzzle space you created. Make sure the stone is below mowing level.