Remove the cover from the new breaker panel by loosening the four screws securing it. Identify the mounting holes on the inside of the box. If you are unsure which holes are the correct ones, consult the owner's manual for the panel.
Hold the breaker box against the wall with your hands. Place a level on top of the box and adjust it until the bubble is in between the two lines on the level.
Place a level on the side of the box to double-check that it is straight. Make adjustments if the bubble on the level is not between the two lines.
Secure the panel to the wall with wood screws through the mounting holes on the inside. Tighten these screws until they firmly hold the panel to the plywood.
Give a firm pull on the panel to ensure it is secure.