Measure the total vertical distance the stairs will cover. In order to do this, lay a board on the porch and measure from the ground to the top of the board. This measurement is called the total rise.
Divide the total rise by 7 inches---the usual rise per step---and round up or down to get a whole number. For instance, if the total rise is 67 inches, divide 67 by 7 to come up with 9.57. Round this number up to 10 and you have the number of steps you will need to use. To find the actual rise, divide 67 by 10 and you get 6.7 inches per step.
Multiply the number of steps you have by the optimal run, which is 10. The optimal run is the ideal amount of space each step covers in horizontal depth. In this example, the total run is 100 inches.
Attach stair gauges to the framing square and mark the tread notches with the framing square so that the riser is 6.7 inches and the tread is 10 inches. Lay the square on a piece of 2-by-12, and make your markings in the shape of stairs. This allows you to have a measurement of where to cut the stringer. It is vital that you ensure the lines are measured correctly and plumb so the stringer is not uneven. Repeat this step with two or three additional pieces of 2-by-12, depending on the width of the steps.
Cut the notches on the stringers with a circular saw, and finish with a handsaw, making sure you do not go past the marked lines.
Attach each stringer 16 inches from one another onto the deck frame with screws. Check that each step position is level, and shave down areas that are too high with a block plane.
Cut enough risers and treads to fit with the number of stairs you have, ensuring they overhang the outer portion of the stringer by 1 1/4 inches. Fasten the risers to the stringers, using 2 1/2-inch decking screws, and attach two treads for each step, leaving 1/8 to 1/4-inch between each tread.