Corner shelves can be made in an "L" shape with one piece being attached perpendicular to the other, with both attached to two brackets on each side of the wall corner. Or the shelves can be made of four equal parts cut from a circular piece of plywood--like four pieces of a pizza. Though both types can be installed with regular wall brackets, the second "quarter-circle" types can have 1-by-2-inch strips of wood connected to the wall corner, with the shelves glued and nailed to the strips.
Mark the very centre of the wall corner at the height you want to hang the shelf. Place the top edge of a level against the mark on one side of the corner and run a pencil along its top edge. Do the same on the other side of the corner.
Run a stud finder along the pencil lines on both sides of the corner, marking the wall stud centres on the lines.
Rest one bracket over a stud mark, making sure the bracket top is flush with the pencil line. Use 1 5/8-inch screws to attach the bracket to the wall. Repeat this with all brackets.
Place the shelf on the brackets with its outside corner pushed into the corner of the room. Use 1/2-inch wood screws to attach the shelf to the brackets.
Mark the corner of the room at the height of the shelf, and run level lines out from the mark on each side of the corner--use the level.
Measure the width of the shelf and cut one piece of 1-by-2-inch wood to this length and another piece the same length, less the width of the first piece. For example, if the first piece is cut 12 inches long and is 3/4-inch wide, the second piece will be 12 inches less 3/4-inch, making it 11 1/4 inches long. Sand the cut edges to remove any splinters.
Drill two screw holes into each piece of wood--use 1/8-inch wood bit--two inches from each end, and in the centre.
Hold each piece of wood against the lines on the wall, marking the position of the screw holes: Create holes at each mark using a large nail and insert wall plugs into the holes.
Attach the wood strips to the wall plugs with 2-inch wood screws. Then place wood glue on the top of the strips and push the shelves down in position onto the strips. Also use 1 1/2-inch finishing nails--3 on each side--to attach the shelf to the strips.
If studs can't be found in the right location for the "L"-shaped corner shelves, follow Step 4 of the second section to insert wall plugs into the wall to attach the brackets.