The new skirt board must be long enough to reach from top to bottom of the staircase run and wide enough to span the tread width and riser height. Typically, a 1x12" board is used. Because a staircase is often a showpiece fixture in the home, choose a quality board such as A-grade "clear" (no knots) lumber stock.
Position the board against the edge of the staircase and temporarily secure it in place.
Make a vertical mark where each of the risers meets the skirt board. Also make a horizontal mark on the board for each of the tread tops.
Remove the skirt board and lay it on a bench to readjust the marks. Move the line for each tread down one inch so the finished skirt will fit under the existing treads.
Use a circular saw to cut the riser and tread lines at a standard 90-degree angle. Start with the power saw, then finish the cut with a hand saw or jigsaw.
Test fit the skirt board against the side of the staircase. The board should fit snugly below each of the treads, and the riser cuts should be flush with the face of the original risers. It is most important that the riser cuts line up – the tread cuts don't have to be as accurate because cove moldings are typically installed below tread nosings and cover any gaps.