Place the carpenter’s square against the stair’s stringer. The stringer is the diagonal board that rests against the wall. Mark a 90-degree-angled straight line at the top and bottom of the stairs with a pencil.
Measure from an equal point on both lines from top to bottom. This is your stringer length.
Measure the height you want the first floor’s wainscoting to be. This is calculated by adding the height of the first step to 32 inches. So, if your first step is 3 inches high, then your first floor wainscoting height is 35 inches.
Measure the rise and run of the stringer by drawing a 2 foot straight line horizontally from the top of the stair’s stringer. Then draw a vertical line from the end of the 2 foot line to where the stringer falls.