Set a level against one wall of the corner in which you want to install your shelf. Shift the level up and down as needed until you reach the appropriate height of your shelf. Trace a pencil along the level to create a straight line at which you will install the shelf.
Shift the level to the opposite wall and trace a level line at the same location. This is the installation guideline for your shelf.
Hold up the shelf at the wall lines and mark the edges of the shelf vertically on the wall, so that the two edge lines intersect your installation guideline.
Measure to the mid point between the corner and each edge mark and make a pencil mark on the guideline. This the best place to install your support brackets.
Drill a small pilot hole at the center mark for each line. If you are installing in masonry (brick, cement, etc.) you will need a special carbide drill head; otherwise you can use a regular drill.
Drill a plastic screw anchor into the pilot holes using your drill. These anchors hold extra weight for the screws.
Line up the shelf's support brackets with the anchors so that the hole for the screw sits over the anchor. Hold it level and insert the screw, then drill it into the anchor so that it supports the bracket.
Repeat bracket installation with the second bracket on the other wall. Set the shelf on top of the brackets and tighten as necessary until everything is secure. Some manufacturers may require you to attach the shelf to the brackets; follow instructions as necessary.
Erase any remaining pencil marks on the wall.