Most stair stringers can
be cut with a 7 1/4 inch circular saw. I remember the good old days,
when I used to spend all day long, day after day, cutting stair
stringers, for tracks of homes, with over a hundred stairways.
Trust me, I got pretty good at cutting stringers and stacks of plywood, for stair treads and risers. Remember the old saying, practice makes perfect, well I definitely got enough practice.
In the picture above the cut stair stringer is sitting on top of a 2 x 4 block. This separates the stair stringer that you're cutting, from the bench that you're working on.
I usually start at either the top or the bottom, of the stair stringer and work my way forward, in one direction. From either right to left or from left to right. I will either cut all of the risers or all of the treads first and then switch directions, working my way back, until the stringer is finished.
There is no reason to ever back up, while cutting your stair stringers.
This is a must, ensure that your stair stringers are sitting on a flat surface that you're comfortable working on, otherwise transfer the stringers, to a better location, before cutting.
Always work on a safe and clean surface.