Attach the end of the rail to the header bracket above the door per the instructions that come with your door opener. Once you install the rail to the header, the opener is at an angle with the opener motor on the floor.
Pick up the opener motor head and raise the motor above your head while an assistant slowly opens the garage door. Gently lower the motor so that the rail is resting on the open garage door.
Position a stepladder next to the door opener motor. Move the motor left or right until the rail is at the center of the door width. Lift the motor enough that you are able to place a 2-inch-by-4-inch board on its side between the door and the opener rail. Gently lower the motor until the rail is resting on the board.
Slide a stud finder across the ceiling above the door opener motor and find two floor joists, one on each side of the opener motor. If the joists run from side to side, find one that is closest above the opener motor. Mark the joists with a pencil.
Measure between the joists, if the joists run front to back. Cut a piece of pre-punch angle to the measurement with a hacksaw. If your joists run side to side, cut a piece of pre-punch angle to a length of16 inches.
Place the angle against the ceiling above the opener motor and secure it with 3-inch wood lag screws and a drill with a socket adapter. The ends of the angle should be even on each side of the opener motor. Measure from the angle down to the rail on each side of the motor that has the holes.
Cut two pieces of pre-punch angle to your measurement. Hold one end of one piece of angle onto the ceiling angle. The second piece should be straight up and down and against the outside of one side of the opener motor.
Insert a 1/2-inch bolt and nut through the end hole of the second piece of angle and a hole in the ceiling angle. Do not tighten the bolt and nut at this time. Insert another bolt and nut through the opposite end of the angle and one of the holes in the top flange of the opener. Repeat for the opposite side of the motor.
Measure from the first open hole on the left side pre-punch angle to a hole near the center of the right side angle. Mark the two holes with a pencil.
Cut a piece of pre-punch angle to the measurement. Attach the pre-punch angle with bolts and nuts at your marks. Tighten all of the nuts and bolts with a wrench and socket wrench. Remove the piece of board from the top of the door.