Move the fireplace surround in place and put a level on top of the mantel. If your floor isn't level, the fireplace surround won't be level, either. If this is the case, trim one of the fireplace surround's legs with a jigsaw so the mantel is level.
Trace the outline of the fireplace surround onto the wall, using a pencil. Move the fireplace surround away from the wall.
Measure the distance from each side of the fireplace surround to the wood strips on the back of the fireplace surround. Mark the corresponding distance on the wall.
Attach the wood strips that came with your fireplace surround to the wall at the spots you marked in Step 3. Using a power drill, drive 3-inch drywall screws through the wood strips into the wall studs.
Spread construction adhesive on the wood strips. This will attach them to the wood strips on the back of the fireplace surround.
Move the fireplace surround in place, using the pencil marks on the wall as a guide. Drive screws through each of the predrilled holes in the fireplace surround, attaching it to the wall studs and the wood strips.