To support the ceiling panels, several scab boards that provide nailing surfaces will be needed. Typically, areas that require scabs are along the end truss at the rafters and in the triangle area (16" centers), and the full length of the inside peak on each side.
Use a straightedge to mark locations and then snap chalklines. Screw or nail on the 2x2 or 2x4 nailers over the lines, but make some extra marks to show where the nailers are when the panels cover them up.
Pre-finished 4x8 plywood paneling sheets are a good choice for the interior of a porch. Run the seams vertically, and fasten them with colored nails that matched the plywood.
To fill in the triangle-shaped area at the gable, use 4' sections (width of sheet) on each side (if possible) and fill in the remaining area with cut-to-fit pieces. Run a plumb line from the peak to the bottom of the header and a second plumb line 4' over.
Measure and mark those heights and transfer them to the back of the panel. Then draw a straight line connecting the heights to find the roof angle. Cut from the back to minimize chipping.
Toenail 2x4 nailers to the house end rafters and secure them to the truss with screws. Firm up the panel edges by screwing in 2x4s between the nailers. For extra stick, run a bead of adhesive along the nailers.
Cut locations for electrical boxes by transposing their locations onto the panels. Drill pilot holes to start the cuts and finish them with a jigsaw.