Cut the stringers. Cut three stair stringers from 2" x 12" lumber to accommodate a 3-1/2" stair height. Cut a notch in the bottom of the stringers to fit over a piece of 2" x 4" lumber, making sure to cut the notch in the same place on each stringer.
For each stair, cut 10" treads from 2" x 10" lumber and 7" risers from OSB sheets. Also, cut one piece of treated 2" x 4" lumber to 3' long.
Note: These stairs will rest on a bare concrete floor -- but untreated lumber will absorb moisture and rot from bare concrete. To combat this, install the treated 2" x 4" directly against the floor.
Position one stringer against the wall and have a helper hold it in place. Fit the treated 2" x 4" into the notch and press it flush to the wall. Use a masonry bit to drill through the 2" x 4" and into the concrete floor, then attach the 2" x 4" to the floor with concrete nails.
Toenail the first stringer to the 2" x 4" and attach to the top stair support with metal brackets. Attach a second stringer in the same way at the other end of the 2" x 4". Attach the final stringer halfway between the other two.