Here is where breaking the mill glaze on a wide board really pays off. The character of the wood really comes across without any interference. As per the owner's request the tighter grain is above and the more open below toward the treads and risers.
In working around older manufactured stairs which are subject to shrinkage, warping or twisting trying to match their contour is extremely complicated if one assumes the course of measuring each tread and riser individually. It is better to not measure them at all! How does one do this? It's easy, hot-glued plywood strips. I cut 1/4" plywood into strips. In this case I make three types: one for the tread that starts inside at the riser and goes past the nose a couple inches, another that fits inside the riser space average by a fraction, and another that rises further than the tread piece, tread to tread and places the nose's exact location. These are all hot-glued together rather simply and braced with a rigid board so the whole assembly will not distort when it is pulled off. The short version of this method (the right side of the stairway as one goes up) of patterning the upper stringer I've show below.
Trim applied to top stringer.