Make a mark on the wall at the location where you want the center of the shelf to be.
Place a level at the mark and center the bubble. Measure out from the level and make marks at 10 and 25 inches from each side of the center marking.
Place a shelf mounting bracket at each of the locations and mark the mounting screw hole locations on the wall with a pencil.
Drill holes into each of the marked locations and insert the proper wall anchor, depending on the wall's construction.
Line up the brackets with the wall anchors and attach the brackets with screws through the mounting holes and into the anchors.
Place the shelf onto the brackets. Attach the shelf to each bracket with a single screw through the hole in the underside of the bracket.