Measure the width of the fascia board on the vertical edge of the roof overhang with a measuring tape from the bottom of the board to the top where the bottom course of shingles overhang it.
Transfer the fascia board measurement to the widest side of a length of vinyl fascia cap. Make a mark at both ends of the cap. Connect the marks at each end by snapping a chalk line between them.
Cut the vinyl fascia cap to width on the chalk line using tin snips.
Hold a vinyl fascia cap -- which has a cross-section in the shape of an "L" -- into position on the fascia board with the cut side facing the top. Ensure the narrower of the two edges on the cap is at the underside of the fascia board.
Slide the upper edge of the cap under the roof drip edge until the underside of the cap is flush with the underside of the fascia board.
Secure the cap to the fascia board with color-matched aluminum nails spaced every 16 inches and driven in with a hammer. Be careful on the last strike that you do not damage the cap with the head of the hammer.
Install another length of vinyl fascia cap in the same fashion, overlapping the previous piece by 3 inches. Cut the last cap at the end of the fascia board to length with tin snips.