Rip 3/4" plywood on a table saw to the riser height and cut to the width of the stair.
Nail a riser on each upright face of the stringers – all the way down.
Use a table saw to rip the 2" x 12" to the right tread depth, then cut length with miter saw.
Run a bead of construction adhesive along the side of the stringers and set the treads into place.
Nail treads in place using #8 framing nails (Image 1).
Nail boards atop landings (Image 2).
Next, add winder landings to make a 90-degree turn. First, build three 30 degree angled boxes out of 2" x 12"s ripped to the riser height.
Miter corners by marking angle on wood with a T-bevel, then start miter cuts on each side with circular saw.
Use a reciprocating saw to finish cutting through 2" x 12" at angle.
Nail triangular boxes together with #16 framing nails.
Run a bead of construction adhesive along edges of third landing where first winder landing will sit.
Toenail it into position.
Add second and third winder landings in the same way.
Cut triangular treads to fit the tops of the winder landings out of 2" x 12"s.
Where the tread is wider than 12", join two pieces together with a cleat on the back made out of 3/4" plywood screwed into place (Image 3).
Use glue and nails to attach the winder landing treads into place.