After much experimentation, watching video tutorials, etc., I have finally figured out one way to do stonework that I find works well for me. I glued pieces of "dollar store" foamcore (the advantage it has is the paper on either side of the foam is easy to peel off) to some of my basic building structures. Then I cut in the stonework with a sharp hobby knife. Then I traced over all of the cut lines with a ballpoint pen to deepen them and to separate the individual stones more. I used the scrunched up ball of aluminum foil trick to imprint some texture on the stones. Then painted the stonework with gray gesso, followed by a light wash of black ink mixed with water. Then painted some of the stones in various shades of gray. And finished off by sponging on a light gray.
I made 2 low stone staircases, and small archway, a building "riser"/foundation, and a full stone building.