If you are using ARCHICAD 21 it's easier, but if you are still using ARCHICAD 19 you can create a fill in the plan view with the projected shape of your stair, then draw with lines or arcs the walk-line of your stair, select fill and lines and go to
you can also create it by creating with this tool piece-by-piece your fights and landings, but it will bel a little difficult to create the handrail.
