The construction was fairly straightforward table saw work. The edges were cut from a normal pine 2x4 you can get at the big box stores for 2$. I used two of them for this project. The top and bottom was made from 1/4 inch sanded plywood. I used a 4x4 ft section of plywood for all of these shelves and had some left over. The shelves vary in length but are all 6 inches deep from wall to edge.
I cut the 2x4s roughly to length on the table saw, then used my joiner on two sides of each piece to make them straight, then ripped it on the table saw to size using the joined edges as reference. Here's a video from somebody else (The Wood Whisperer) on how and why to do that.
Before cutting the corners to fit together, I cut the rabbets for the plywood with two cuts on the table saw. The rabbets are the grooves that allow the plywood to sit flush with the top of the edge board.
I then mitered the corners to fit together at 45 degrees, cut the plywood to fit for each shelf, and then glued them together. The plywood was glued everywhere it touches the edges. The miter corners are also glued together. Instead of clamps I used a pneumatic brad nailer and 5/8 inch 18 gauge brads on the top and bottom. I did not shoot any nails into the corners because I wanted to do the final bevel and routing after assembly.
After the glue set (an hour or so) I cut the edge bevel and ran the router along the bottom edge. The brad holes and any other gaps were filled in with wood putty and then sanded. If I was going to stain these instead of painting them, I probably would have used clamps instead of brad nails to avoid the holes and putty. Here are pictures of how the plywood sits in the rabbets.
I used a micrometer to measure for brackets to hold the shelves to the wall. The brackets are just a chunk of wood that fits exactly inside the shelves top to bottom. I cut them to have some slack left to right to make assembly easier. When the shelves are installed these are completely hidden. Here is a picture of a bracket fitting inside of the shelf.
I took the brackets, a pencil, and a stud finder into the house where the shelves would hang. I marked the stud locations on the brackets then pre-drilled and counter-sunk some holes in them. Since the brackets are hidden I was free to use any size screw or bolt to secure them to the wall. Three inch deck screws into the wall studs worked fine for me. Drywall expansion fasteners would work great too.
When I screwed the brackets to the wall, I made sure they were level.
Then it was just a matter of sliding the painted shelf onto the bracket, pre-drilling a hole, and sinking a screw to hold them on.
With the brackets solidly mounted to the wall, I think these shelves will break apart before they ever droop or fall down. I hate droopy floating shelves and like this system more than the floating shelves you can buy. If I really want it to look clean, I might paint the screws, but for now they'll be hidden behind picture frames. The overall cost of construction was less than 5$ per shelf and all six only took three or so hours total construction time.