The stairs were trial and error, and also cut the bejeezus out of my finger trimming off rough edges. :/
I have several surplus sets of stairs and their accompanying upper floor panels with the hole for the stairs. From the larger end of one of these, I cut the landing to be the same width and length as the two pieces of stair, plus a bit.
I cut a set of stairs at the desired point, removed a single stair from the shorter piece so the angles looked right, and attached it to one edge of the landing using cyanoacrylate and/or Plasti-Weld. Legs for the opposite edge of the landing came from the two thin edges of the floor panel. They support the bottom edge of the upper stairs and are glued in place, so the landing and lower stairs are freestanding.
Stair railings are trimmed sections of longer original pieces. They disguise some of the sins of creating without a plan. All the railings on the landing and lower stairs are glued in place.
In a couple of places I had to add little wedges of support plastic because I could not cut the angles I wanted safely with the saw I am using, so those wedges are plastic-welded in place.
As the second floor is set up to be open plan, these stairs will go there. Otherwise they stick out too far into the room.
A fun little project. Would do it again if the house had space for more.
