It's a very simple procedure.

1. Remove your existing cap.

2. Remove & keep the 4 screws that you see. They
hold the filler neck to the inside of the 1/4 panel.

3. Place the trim ring over the opening, so that the 4 holes in the trim ring line up with the 4 holes where you removed the screws. The depression on the ring goes to the rear of the car, and the "flipper" handle on the cap goes in the depression.

4. Place the flip cap over the trim ring, again lining up the 4 holes.

5. Replace the 4 screws.

6. Find an inner cap to actully keep the gas from sloshing out. The flip cap is nothing more than decoration.

7. You are done...

One note: The directions above are for if you have a factory 1/4 panel. If you replced the 1/4 panel with an aftermarket one, all bets are off. They are notorius for having an improperly shaped opening for the cap. If that's the case, good luck. I had that problem, and it required some "re-engineering".