All items must be made as boxes except the balusters which are cylinder-shaped.
If not told otherwise the stairs are 900 mm wide including stringers on both sides.
The stringers are 50mm wide
Tread-box is 10mm deeper than the riser-box, so that you get a “stair nose” which is 10mm. The riser covers the back side of the tread, and the shape is like shown in illustrations below. The first is a schematic view from the side and the second shows how it looks in the planner.