Installing Soffit and Facia???
I believe you are asking how to put a level return on, commonly called a soffit or cornice return. Basically, you attach a triangular 2x to the bottom of your fly rafter on the gable end, (bottom level, hypotenuse cut at the same pitch as the roof) using nails and construction adhesive, then run a 2x6 from there back to the house sheathing on the eave side, (level with the bottom of the eave fly rafter. If using F- channel this would go on the 2x6 the depth of the soffit then this is wrapped with fascia first before the gable ends are done, work from the bottom up. Takes longer to explain then to actually do.