Place the back wall adjacent to a piece of plywood and draw the gable shape. Mark the angles (approximately 60°) on the rafters and make your cuts.
Join the rafters at the peak and to the top plate. Use exterior glue and galvanized screws. Make the job easier by pre-boring all screw holes and by clamping the workpieces together prior to driving the screws.
Mark for the two angled cuts on the door header and make the cuts. Then glue and screw the header to the rafters.