Swinging a tape from a wooden stake, the author lays out the inner and outer arcs of the stair at full scale on a double layer of plywood.
Then, he swings the walk line a foot inside the inner arc. Tread locations are marked at 10-inch-minimum intervals.
Chalk lines snapped from the stake through the tread marks on the walk line define the size and shape of the treads. The author verifies that the minimum tread width is at least 6 inches.
After scribing the stair on the plywood on the ground, the author makes a mirror image of the layout from a double layer of plywood, adding 3 1/2 inches — a stud width — to both edges.
Straight, wane-free studs screwed to the plywood templates behind each riser line provide a bending form.
He then installs the stringer laminations, aligning the top edges with the marks and gluing and screwing them together.
The author makes initial cuts with a circular saw and finishes up with a recip saw to avoid overcutting the inside corners.
The risers are made from two layers of 3/4-inch plywood lapped so as to support the front and back edges of the 2-by structural treads.