The tricky part of the outside stops and trim will be the bottom section. The old bottom jam or sill is probably sloped to shed water. You may have to rip a 2 x 2 on an angle to match the slope to use for the stop on the bottom. The bottom of the replacement window will not be angled either, so you may have to cut special shims to support the window properly. Another way to get this job done is to contact your public power company and ask about their window and insulation upgrades.
Many power companies have funding for low interest loans or even free upgrades.
For the risers measure total rise in inches, divide that by 7.5. Divide the total rise by the number of risers. This is the rise per step and should be between 7 and 8 inches. If you can add a riser to stay closer to 7 inches, do it. For the run Measure the total run in inches and divide by the number of risers.