Once your glue is dry, lay the panel screwside up flat on your bench, and mark the center point across the bottom edge (it should be at 8-1/4″). Now, using an offcut as the base for the compass point, measure 2″ down from the center point, place the compass point there, and draw the arc. Use a jigsaw to cut out the arc (you’ll be cutting away the two bottom screws), then clean up the cut using a rasp and sandpaper.
Use a miter saw to cut the three step supports and back brace to 15-3/4″ out of 1×2 (which actually measures 3/4″ x 1-1/2″). Clamp each piece flat and drill one pocket hole into the center at either end. Set them aside.
Now, out of 1×8 (actually 3/4″ x 7-1/2″), cut the three step treads to their final length of 18-1/4″, then clamp a piece flat to your bench, with one long edge overhanging. Use your jigsaw to rip each tread to a width of 5-7/8″. (You’ll get a straighter cut if you clamp a straightedge in place to guide the foot of the jigsaw.)
Now all the pieces are ready for assembly; this is a good time to sand (because as yet there are no corners to deal with). Sand over the panel joints to ensure the panels are flat. If you’re planning to paint, sand to #120; if staining, you may want to sand to #180.
Joining the step supports and back brace to the sides is a bit tricky, involving some creative clamping and a scrap of 2×4. Separate the two halves of your Workmate so you can get clamps through the middle, then clamp the 2×4 scrap tight to the table. Butt the front edge of the bottom riser (the short piece in the panel) flush to the 2×4, and clamp the panel to the table. Hold the step support against the 2×4 flush with the top of the riser, gripping firmly to keep it aligned, then drive the screw into the pocket to join the support to the panel (see picture at left). Reclamp the panel against the 2×4 to attach the center and top step supports, as well as the back brace.
Center each tread across the steps, drill pilot holes, then nail the treads in place with #6 finish nails. Use a nailset to sink the nails below the tread surface, then fill the holes with wood filler (or spackle), and let it dry.
Once the filler is dry, sand the joint edges and filler flush. Chamfer the front and side edges of the treads with a block plane to keep them from splintering, and break the back edge with sandpaper. Paint or stain as desired. PW