Making base cabinets out of upper cabinets or wall cabinets is a fairly simple procedure. The two units are very similar in construction with three major differences. Upper cabinets are shallower from front to back, do not have bases and typically are built with a solid top versus the open cleat construction found in most lower cabinets. We will assume that you are wanting the shallower depth, and concentrate on correcting the other two differences for your cabinet conversion.
Things You'll Need
Nails and hammer or compressor and pin nailer
Drill and screws
Adapting the Cabinet
Build a rectangular tray of 3/4-inch plywood 3 to 4 inches deep, as wide as your cabinet and about 2 inches shallower. Stand the pieces on edge and nail the corners, using plenty of wood glue. Add two cleats--3- to 4-inch wide strips of plywood--as wide as the inside of your frame. Tap them to the bottom, flush with the tray edges. Glue and nail into position. This is your cabinet's base.
Place your cabinet upside down with the tray on top, cleats down. Center it side to side so it is flush with cabinet sides and back and about 2 inches back from the front edge. Using screws that are not long enough to pierce the cabinet bottom, attach it firmly to the cabinet. You may need to use 3-4inch plywood as a shim to take up the space between the cabinet bottom and base.
Turn the cabinet right side up and cut two cleats to fit inside the top lip of the cabinet snugly from side to side. Glue and nail these in place. Make sure they are flush with the top of the cabinet.
Installing the Cabinet
Remove any molding or other obstructions from the wall and find and mark wall studs. Set the cabinet against wall and check for levelness. Shim as needed.
Attach using 3-inch screws through back of the cabinet into wall studs. For best results, locate cabinet back cleats and screw through them. Use at least four screws per unit. Screw side-by-side units to each other, then to the wall.
Add a toekick--cut from plywood--at the bottom and install a countertop as for standard lower cabinets. To attach the countertop, use a paddle bit 1 inch wide to drill through the cabinet from inside to access top cleats for easy fastening.