premise: Flushing the bottom of the stringer with ground.
hypothesis: Draw a line parallel to the top of the last step notch, and perpendicular to the side of it (where the bottom riser will eventually go). Cut this away to make the bottom of the stringer flush with the ground. Now test the stringer by setting it in place. Cut the top and bottom of the stringer to fit, then test-fit it before cutting it out. Make sure the height is accurate. Both top and bottom points should be flush before fastening.