Measure the various areas where the door will be installed: door-opening height and width, headroom (from top of opening to ceiling), and back room (length of garage). For the headroom, you should have about 10" to 12" of space, depending on the spring system you're installing. For the back room, you should have the door height, plus another 18" or more of space.
With the proper measurements in hand, temporarily attach stops to both sides of the opening and make sure they are flush with the inside edge of the door jamb. Using the sawhorses, lay out the bottom piece of the new door, and attach any weather stripping (if necessary). Additionally, attach any hinges and lift handles to the piece.