Lay a 1-by-10-inch skirtboard plank on the corners of the treads, with its upper corner beyond the top tread nosing, and its lower corner resting on the subfloor beyond the stairway. Nail this board to the wall so it stays in place while you outline the tread and riser locations. However, do not pound the nails all the way through the wall, so you can easily remove the board and cut along the marked lines to form the skirtboard.
Mark two lines on the wall along the stairway, flush against the skirtboard. Form one line above the upper tread, and the other above the bottom tread.
Hold the carpenter’s scribe vertically against the stair with the highest riser. Form a line on the mark that matches the rise of the stair. Ensure that the scribe is plumb before driving a nail through it at the point that marks the rise. Continue this process to scribe a vertical line at each tread along the stairway.
Remove the nails through the skirtboard, and then remove the skirtboard from the wall. Lay the board on the cutting table.
Cut along the lowermost scribe mark on the skirtboard, using a handsaw.
Position the skirtboard against the wall so it slides down on the floor. Align the skirtboard with the two marks you made on the wall.
Nail the skirtboard to the wall again, leaving a portion of the nail shanks and heads exposed for easy removal.
Place the carpenter’s scribe on the treads so it covers the run or depth of the stairs. Insert a nail through the scribe to secure it to the skirtboard and wall. Mark the corner of the scribe on the skirtboard. Extend this point vertically up to meet the riser of the stair above. Use a ruler to form straight lines on the skirtboard. Continue this process along the stairway.
Remove the scribes and set each horizontally against the risers. Mark the ends of the scribes to transfer the depths of each nosing on the skirtboard.
Remove the skirtboard from the wall. Extend the tread marks on the skirtboard to the nosing marks, using a pencil.
Cut over the tread and riser lines on the skirtboard using a circular saw or jigsaw. Replace the saw with a handsaw when cutting the inner corners.
Slide the skirtboard back over the stairs. Mark the points on the skirtboard where it falls higher than the risers, and trim as needed, so it falls flush against the stairs and wall.
Hold a level plumb against the skirtboard on the top tread. Mark a line on the skirtboard that matches the height of the surrounding baseboard. Repeat the process at the bottom of the skirtboard. Cut along the vertical lines to provide a smooth transition between the baseboard and the skirtboard.