Stairways look complicated, but basically they are a variation of a simple incline which in turn is a rectangular block sliced diagonally in half, right, like the cheese wedge in an earlier sketch. Above is a basic set of stairs.
This is a sketch that I intended to work up as a finished drawing, but never did. Everything sagged in the old Harper's Ferry building, and its linear perspective is far from perfect. My eye level was about in the middle of the right-hand steps. The upper landing sagged so much that instead of seeing slightly up under it, I could see the tops of its boards.
I'll take you step by step through the process ...