Spiral stairs are easy when you know how...
Have a look at my video on design a base using World Edit.
It includes making a spirl (helix) including the slope for placing stairs.
I even show the results after I built it in survival without world edit.
Ants 026 World Edit Base
If however you just want a flat spiral staircase (using slabs) which I used in a different base. See the older video
Ants 019 Spiral Ramps