Hi Dan: Although I've done many times what you are trying to do, it may be a little tough to explain. Your local library should have several how-to books that will provide detailed explanations and nice pictures to help you with this. But I'll take
a stab at it anyway.
You don't mention the pitch of your rafters or their centers, but assuming its a 4/12 on 16" centers, cut the ends vertical to the ground, all equadistant from the garage wall. Run a horizontal plumb line from the bottom of the raf
ters to the garage wall and install a 2X4 ledger with its bottom edge on this line, from the front to the back of the garage. I them frame in 2X3 soffit joists from the rafter to the ledger, cutting each joist at the angle necessary to fit plumb with the
bottom edge of the rafter end. I predrill each joist and attach to rafters with 3" drywall screws, as these are a giant pain to nail. I then sheath with 1/2" CCX exterior plywood (no knots showing), and cut the openings between the rafters in the soffit f
or aluminum (or vinyl) screened vents. The facia board should be clear (or #2 for a garage) 1X8 pine. The reason for such a wide board is to ensure you have enough room to mount your sloping gutters. After priming and painting, I run a bead of caulk behin
d the facia board where it meets the 1/2" plywood soffit, just to make sure the little flying insects stay out.