Cut wood to lengths needed - see blog post for wood cutting instructions to fit your area.
Create a box with 1x4's, leaving one of the long sides of the box open for mounting on the wall. Fill nail holes, sand, and stain the box.
Create cleats with 2x2's, securing short pieces of 2x2s to a long piece using two 3.5" screws for each piece
Attach the cleat to the wall studs using 3.5" screws.
Slide the box over the cleats and secure from the top with screws into the back rail piece of the cleat.
Notes
These floating shelves are quite self explanatory once you get into building them. Take your time, follow the blog post, and you'll have a beautiful DIY floating shelf in no time!