To get proper measurements for the skirt boards, you'll need to clean up the stringers first. Cut away the overhang of the landing with a circular saw and hand saw, and remove any leftover frame pieces with the reciprocating saw. Trim up the drywall edges and remove any dried glue. If the new treads are a different thickness than the ones removed adjust for the difference on the stringer. Measure the distance from the floor to the landing and divide by the number of steps. This results in how tall the risers need to be. For this particular project, the stringer treads need to be shimmed 1-1/4".