Measure the desired distance from the sides of the board. Then mark the halfway point of the thickness of the board. Where those two marks meet is where you should drill the hole for the bracket post.
Place the drill bit with the painter's tape into the drill, and line up the point with the X. Check the bubble level on the top of the drill to make sure your bit is level before starting to drill the hole.
Poke through the tape with a pencil where the bracket holes are located, then remove the tape. Then place the tape on the walls where you want the shelves to go, making sure it's level.