Cut the desk rails (L) to rough length, then cut and notch the wall bracket (N). Glue and screw the third support cleat (M) to the front of the wall bracket. Use a sanding block to ease all sharp corners and edges on these parts and on the storage bench assembly. Apply a coat of latex primer to all surfaces, followed by two coats of gloss or semigloss latex enamel. Let dry.
If necessary, prep the alcove area by removing the baseboard molding where the bench will fit. (You may have to cut and reinstall some sections of the baseboard.) Also, mark the locations of wall studs in the alcove area. Fit the bench into place and secure it to at least two wall studs, using 2-1/2-inch screws driven through the back panel (G). Test-fit one of the desk rails (L) against the wall, mark stud locations, then drill 3/16-inch holes and screw the rail firmly to the wall. Similarly fasten the wall bracket (N) to the adjacent alcove end wall, then use glue and finishing nails to attach the front desk rail as shown.
Recheck the dimensions for the desktop/seat platform (O), then cut and test-fit that part in place. Adjust as necessary, then prime and paint. When dry, secure to the supports and the storage bench with finishing nails. Cut the cork tiles to fit and attach to the step platform.