Lay the first board on the shelf brackets. Put it right up to the corner of the walls to determine where the miter cut will start. Use a chop saw to cut the board at that exact angle. Repeat this for shelves above and below.
For the second side of the wall, lay the boards on the shelf brackets. Then lay the already mitered cut boards over the straight-cut boards and draw a line from the mitered board onto the straight board to get an exact match.
Paint or stain the boards as desired. Let them dry completely. For extra stabilization, attach the shelves together at the mitered corners, use flat 4-inch brackets and screw into place on the underside of the shelves.
Place finished boards on the shelf brackets and attached them from the underside of the boards.
Joanne Palmisano is the author of Salvage Secrets (W.W. Norton, September 2011). Visit her blog, also called Salvage Secrets.