Answer: If you have never done any stonework before, it might be a good idea to consult with someone who has, or at least get a good reference book. I've done a little myself, and it can be a challenge. There's a great book called "Stonework: Techniques and Projects", by Charles McRaven (Rodale Press, 1997) that gives lots of information and step-by-step (no pun intended!) photos. Some of their tips for stairs include: Don't build steps too steep. Allow a way for water to run off without undermining your work. Don't leave depressions in the step where water can collect and freeze--tip stone slightly or chisel a groove.
If you are contemplating risers, then it sounds like you have a "stairway" in mind, rather than some simple stones set into a hillside. Rather than using a riser, I think I would try to build up each step to its necessary height, a little stone wall, if you will. I'm afraid that if you use a flat piece as a facing for your riser, it will become unstable after a rain. You need to be concerned with frost heaving, too. We build several low stone walls and a small patio last summer, and all were damaged to some degree by frost heaving. It would be a shame to do all that work this summer and have it heave out of place next winter. Depending on your soil type, you may want to do some site work, perhaps laying a bed of gravel under each step to facilitate water runoff. I'd start from the bottom and work up.
I hope this is helpful to you. Good luck!
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