Measure from the top of the landing platform your steps will rise to down to the floor that the base of the steps will rest on. Divide the measurement by 8 inches. Divide the remainder by the whole number of 8-inch segments. So for a 36-inch platform, you have four full segments and 4 inches remaining. Add the divided remainder to the riser height. For this example, a four-step, 36-inch platform would have 8 1/2-inch risers.
Subtract the riser height from 18 inches to determine the tread depth. For a 36-inch platform, you would have a 9 1/2-inch tread. Wrap a piece of tape around your framing square on the long angle of the square at the dimension of the tread and around the short angle of the square at the dimension of the riser.
Lay a 2-by-12 out flat. Measure and mark the height of your riser from one end of the board on its edge. Position the square so that the taped marks are aligned with the edge of the board. Position the taped mark for the tread on the mark you just made, and the taped mark for the riser on the edge of the board toward the unmarked end of the board. The point of the square should extend into the board to create a 'V.'
Draw along both angles of the 'V' onto the face of the board. Shift the square along the board until the taped tread mark is even with the mark you just made for the top corner of the riser and mark a second 'V.' Continue shifting the square and marking V's for as many steps as you need. You should mark one less than required to meet the top of the platform.
Use the square to continue the top tread across the entire board. This will be the top cut of the stringer. Measure from the front edge of the top riser the distance of the tread dimension. Draw a line from that point, using the square, down to the edge of the board. This will be the mounting point.
Subtract the thickness of your stair tread material, typically 1 ½ inches, from the riser dimension. Draw a parallel line below the first tread equal to the riser height, minus the thickness of the stair tread, completely across the board. This will be the bottom cut.
Cut the board off along the top and bottom cuts with a circular saw. Cut out each 'V' for the riser and tread pattern. Create a second, identical stringer using the first stringer as a template. Cut one tread board for each tread on the stringer as wide as you want your steps to be. Rip the treads to the width of the tread cuts with a table saw.
Position the treads with the ends flush with the outside faces of the stringers and the front edges flush with the front edges of the risers. Drive 16d nails down through the step treads, two nails through the ends of each tread. Cut a 2-by-4 to fit between the stringers and attach it to the top end of the stringers flush with the mounting point.
Set the steps up with the 2-by-4 mounting board against the wall with the top step parallel with the top of the platform the distance of one riser below it. Screw 3-inch treated deck screws through the 2-by-4 you installed between the stringers into the face of the platform. Cut 2-by-4 legs to fit between the floor and bottom of the top step. Screw them in place through the outside of the stringer, against the back of the mounting board.