I measured the studs for the individual pony walls directly from the story pole, and stacked and numbered them for quick assembly.
I installed the support cleats for the backs of the treads as I went, constantly checking for level.
Once the walls were in place, I cut the treads from 3/4-inch subflooring, taking the dimensions from the original OSB pattern.
By using adhesive and staples and lots of pressure, we got the plywood to conform to the arc. I ran it long at the top, then clamped on a strip of 1/8-inch oak for scribing a fair curve about 4 inches above the tread nosings.
I cut 2x6 studs, beveled at the top to match the slope, and glued and nailed them to the elliptical plate at about 6 inches on-center where the radius was tight and 8 inches on-center elsewhere.
I blocked between these studs, using both glue and nails, and added solid blocking at top and bottom to provide anchor points for the metal railing.