In exchange for the great advice I've received on this board, I'd like to offer a simple procedure for anyone looking to cut a skirt board around an existing stairway.
After notching out the treads to the thickness of the skirt board I started by cutting 2 pieces of plywood 3 inches wide and fastened them on the ends to create a 12' long board. Then I placed this board into the stairway and screwed it in place with a temporary fastener.
I cut 12 pieces of cardboard (one for each step) 10" wide by 6.5" which is the height of each riser. Then I aligned each piece of cardboard to each step and stapled it to the plywood.
Once all of the pieces of cardboard are in place, remove the template, trim off the excess cardboard and there you have it, a perfect skirt board template, and it took less than an hour to do.
Hope this comes in handy for someone taking on this project!