I decided to order premade risers from the Canadian company Stairframe. This is how they arrived.
They came with detailed instructions and relevant hardware - screws and everything.
The run I was assembling required two lengths of stringer to be joined on each side.
Here they are assembled (it took about 10 minutes), and resting in the original hole in the ceiling - where we had originally thought to put drop down attic stairs.
I made a new hole, over by the south wall of the garage, and framed it in. The left riser will be bolted directly to the wall.
I had to move the plug that was there up a couple of feet.
The stairs required that last step to be a little taller than the trusses were deep. I hung 2x12's off of the framing to support the top step.
The top step sets into special brackets provided by Stairframe. This was the only (minor) complaint I had about their system. The nuts they provided were a whisker too big to press into the slot on the bracket. I had to grind them a little smaller on my grinding wheel. (Not a big deal except the bearings on my grinding wheel gave out while I was doing it.)
I noticed, after I started assembling the steps, that the track for the garage door would be right about at the middle of my head when I came down the stairs. Fortunately there was a foot or so extra run on the track. I cut it back, and re-hung it so that it was very close to where the roller came to a stop when the door was open.
When I measured the height of the stairs, I forgot to include the calculation that the garage floor was sloped. I ended up with more shim than one would expect. I put these pavers at the bottom so the last step wouldn't be quite so much of a doozy.
I used rough cut 2x6 and 2x4 poplar pieces for the stair treads. I got them at Almandinger's Sawmill in Johnstown. The pieces were not the same thickness, so I shimmed the 2x4's to be flush with the 2x6's.
Here they are assembled, with stuff already stored under them. I still have to attach the railing (which we salvaged) to the wall.
I decided to put that one vertical support and a railing of sorts on the open side of the stairs as well. I will probably go back and assemble more of a railing later, after the attic space is more finished.