Planning and laying out the cut markings for stair stringers is pretty complex until you have done it a couple of times. This job absolutely requires the use of a carpenter's square.
If you are replacing existing stringers, you can cheat by simply using the old rotten one as a template for the new one. The carpenter's or framing square is a simple hunk of steel but one of the most complicated tools you own. There are entire books written on the subject and any one of them would serve as a good primer for laying out stair stringers.
You can purchase pre-cut pressure treated wooden stringers, as well as all metal pre-fabricated stringers, for standard rise & run dimensions.
But the one special thing I always do with outdoor wooden stairs, is to slant them just slightly back towards the porch or deck. That way, if they get icy and your foot wants to slip, it will slide into the next higher step and stop rather than throwing you down the whole flight of steps. This works best with open stairs. If they are not open, make sure that there is a drainage path for water to flow away or you may just be collecting water for an ice rink.