Mark on the bottom landing where the stair will actually end. Measure to determine the stringer and skirt length. Take the stringer or the 2x12 and put them on top of a sawhorse to layout the place where the riser and treads will be. Lay out the stairs by drawing on the outside of the framing square, sliding the square along until it meets the last mark. Remove ¾ in from the top of the stringer to fit the missing top riser. Mark the bottom of the stringer to remove the tread thickness. Cut the top and bottom of the stringer with a circular saw. Fit the stringer into place and check the tread level.