Cut three 2-by-12 boards to a length 6 inches longer than the planned garage door opening. For a 16-foot garage door start with 18-foot boards and cut to size. Nail the boards together, using 16d nails, to form a solid beam about 4 1/2 inches thick by 11 1/2 inches wide.
Lay out the stud wall for the garage. Place king studs, studs that go from the bottom plate to the top plate 3 inches to each side of the planned garage door opening.
Place the header between the kind studs with the front of the header flush with the front of the stud wall. Nail through the king studs into the header ends with 16d nails.
Place two jack or trimmer studs between the bottom of the header and the bottom plate on each side of the planned garage door opening. These studs will support the weight of the header and the roof above it. Nail through the jack studs, with 16d nails, into the king stud to hold the jack studs in place.
Place any other necessary wall studs in place to complete the framing of the wall. These studs are placed at the ends of the stud wall and 16 inches on-center elsewhere in the wall. If the garage door opening takes nearly all the wall space the additional studs may not be necessary.
Lift the wall into place. The weight of the header will require some additional workers to raise the wall. Once the wall is upright fasten the bottom plate to the foundation and use braces to hold the upper portion of the wall in place.