Here I've started to install the risers. These are 5/8" thick scrap plywood (I believe this was subfloor material I removed from one of the rooms. Possibly the living room or dining room). I fastened these with nine 1.5"screws per board. Everything on the staircase will be screwed together, but the staircase itself will be nailed in place. Nails are important in case the house needs to shift slightly. The nails are actually stronger than the screws (screws can snap while the nails will just bend).
In this photo you can see that 2x8 that I laid down across the opening to hold the stringers. Without this board, 2 of the stringers would have been basically floating in the air over the hole. Originally there was some subfloor here, but it was cut back at some point. You can also see how short the bottom step looks, but remember that the final height will be 2 1/4" taller than this.
Same as above, the top step looks much too tall, but there will be a 2 1/4" height difference. Once everything is done, ALL the steps will be 8.5" tall.
Looking down (looks neat!)
Everything seems to line up really nicely.
That's as far as I've got for now. Today I went and bought some large nails, and I shimmed and nailed the stairs in place. I also added 3 metal brackets to support the 3 stringers at the top. I still need to cut and install the rough steps.