Hanging a shelf using 3 wall-mounting brackets: How to align the drill holes?
A helper would really help with a 4’ board


Screw one bracket into the wall (bottom hole only) 6” in from one edge where you would like the shelf
Place the other bracket 6” in from the other end
Place the shelf on the brackets and keep the brackets flat against the shelf 
With the level on the shelf, raise or lower the bracket to find level
Screw in the second bracket in the bottom hole
Screw in the other holes and attach to shelf (ensure the shelf is centered first).
Maybe add a middle bracket, 3’ is a bit of a span depending on what you are putting in it