In the below pics, the left-hand skirt board was my first attempt. After making the floor cut for this skirt, I started second guessing, and measured before cutting out for the treads and risers. Hmmm, got some gaps that aren’t too nice. Good thing I did this intending to fit the risers and treads to the skirt, rather than skirt OVER the treads and risers. It will be okay.
The right hand skirt base (looking from bottom to top of stairs) is my second attempt. NO measurements were taken. I found that making my scribe gauge out of longer stock allowed me to use the same piece of stock and keep it reasonably plumb more easily. I used a scrap piece of poplar. I simply scribed for treads and risers exactly as explained. Worked like a charm! I don’t need a zero tolerance fit because I intend to fit treads and risers after the skirt is on, but this is darned good without any touch up sanding. Very good recap of this technique provided here.