How to set stepping stones in steep, loose hillside
As long as the steps are well drained, you need no foundation.

Dig deep enough that you can lay down at least 4 inches of coarse stone (1/2" to 1" in diameter) below and 4 inches of regular gravel (1/4" in diameter) on top of that (round is better than angular, if you can find it). On top of the gravel use an inch of sand. Place the stone slabs overlapping each other about 50%. So, for example, if each step is 20" deep then shoot for a 10" overlap. The exact dimensions of the steps and overlap will depend on the steepness of the hill. At the bottom of the steps make sure there is some kind of good drainage so that water will not pool there.

Steps of this design will be very solid and last for hundreds of years.