Place the four pieces of 30-inch lumber (stiles) on their long edges. Measure from the ends of the pieces, and make marks at 1 and 2 inches. Put the combination square on the marks and draw lines across the 3/4-inch edge.
Mark the lines in the center of the edge at the 3/8-inch mark. Put the stiles on the drill press table. Drill holes through the stiles with a 1/8-inch drill bit. Drill 3/8-inch countersink holes 1/2-inch deep on top of these holes.
Apply glue to the ends of the 6-inch pieces (rails). Set them between the stiles at the top and bottom. Clamp the stiles to the rails just below the drilled holes. Tighten the clamps until the pieces are snug.
Insert the 3 1/2-inch drywall screws into the holes and secure the pieces tightly. Clean up the excess glue with a damp cloth. Allow the glue to set before removing the clamps.
Remove the clamps and install the adjustable shelf tracks in the center of the stiles 4 inches from one end with the supplied tacks or screws. Next, drill holes as before 3/8 inch from the ends of the 19 1/2-inch pieces (crosspieces). Drill them 1 inch from the long edges. Drill the countersink hole only 1/4-inch deep.
Run a bead of glue on the top and bottom of the stiles. Secure the crosspieces to them with the 1 1/4-inch drywall screws. Clean up the excess glue with a damp cloth.
Measure the distance between the adjustable shelf tracks and subtract 1/8 inch. Cut the shelves to this length from 1-inch by 12-inch lumber. Use a circular saw or radial arm saw (if available).