Construct a one-by-two poplar framework for each shelf to cover with plywood. Cut and miter strips of poplar to face the exposed plywood edges on the front and sides. Secure each component with glue and nails. Spackle the nail holes and then sand and paint each shelf. Wall-mount poplar cleats to studs and seat the hollow shelving on top.
Set up a table saw and fence to cut 1/2-inch plywood skins to measure. Cut an identical pair of panels to cover the top and bottom framework of each individual shelving unit. Units may be of different lengths, if desired.
Cut poplar into one-by two inch strips for the frameworks of all the floating wooden shelves. Rip the strips to identical width so that all the finished shelves will be of uniform thickness when the strips are assembled on edge.
Cut one-by-two poplar for each floating wall shelf's framework. Make a pair of long sides, a pair of short sides, and a pair of cross braces for each, cut to measure so they will fit the plywood skins made earlier.
Apply glue and position a pair of long sides and short sides in a long rectangle of correct dimensions. Secure them with a nail gun, and then glue and nail the cross braces.
Apply carpenter's glue to the edges of the framework. Position the plywood skin and staple through the skin into the framework along its perimeter and the cross braces. Repeat for the other side.
Cut one-by-two facing strips to cover the three exposed edges of each shelf unit. Miter the ends so they meet to form neat corners. Glue and nail the front piece, followed by the end pieces.
Use a putty knife to fill all the nail holes, and then use a small sander to smooth everything to the touch. All surfaces should be ready for finishing.
Finish all exterior surfaces of the shelving units before they are mounted. Work outside in a well ventilated area to minimize fumes from the paint. Apply two coats per manufacturer's directions and allow each coat to dry.
Use painter's tape to visualize the best position for each shelf. Mark stud and cleat positions and ensure they are level before screwing them into the studs.
Pre-drill counter-sunk screw holes in the back top edge of the shelf. Clip the open back over its cleat and tap it in place. Drive screws through the holes into the cleats to secure.