When cutting stair stringers, first decide if the stringers are going to be attached flush with the deck top or if they are going to be attached under the deck. Once that is determined, mark the tread notches with a framing square using stair gauges to ensure notches are identical.
Use the stair gauge to clip onto the framing square's tongue for the rise dimension, and then attach the other gauge on the body of the framing square for the run dimension. Using a 2 by 12 board, lay the square down with the gauges against the edge, and make markings for the tread and riser. Repeat this step by gliding the square down, lining it up with the previous notch, and marking the next notch until you have marked all of the steps.
Use a circular saw to cut out the notches, but do not cut past the markings. To complete the square cuts, use a jigsaw or a handsaw. Once the cuts are made, measure the tread thickness for the bottom of the stairs, and trim the stringer accordingly. Use the first stringer as a template for creating the rest of the stringers to maintain continuity. Once the stringers are in place, use a level on every step, and trim any uneven spaces.