I just finished re-installing my fuel filler door and can attest that it is a miserable job. Now that I have done it, I could probably do it again a lot quicker; much of the time that I spent involved going down blind alleys.
There are two challenges to this job:
1) Getting the door into position. The spring on the hinge is bi-stable: it either wants to be closed or open. And, it won't fit into where it needs to go in either of those two positions. Almost fully open is where you want it to be and I achieved that by wedging a screw-washer-nut into the hinge to hold it open, but not fully open. See the first two pictures below. Note that the washer needed to be filed flat on one side so that it did not protrude below the hinge's mounting surface. I used a #8 screw, and had to grind it flat to fit into the gap below the hinge.
2) The second challenge is getting the securing nuts started once the door is in place. The studs on the body have to be short to allow the door to be squeezed into place, which doesn't leave much stud protruding to start the nuts. Because the hinge won't be parallel to the mounting surface, the most forward stud will be the first one to start. Someone above suggested sticking the little 5mm nut to your finger with grease to maneuver it into place. Instead, I used 30mm long, 5mm threaded connectors - sort of "extra long nuts". These are available on Amazon (
https://www.amazon.com/dp/B0FFMGL354?ref=ppx_yo2ov_dt_b_fed_asin_title&th=1 ) for ~$1 apiece, next day delivery. You can easily grip these with your fingers to twirl them started. Once started, a socket (5/16" worked for me) on an extension will drive them into place. See the first photo below.
One blind alley that I went down: I had removed and then re-glued the elephant skin fabric that covers the inside of the fenders, around the gas door opening. I had mis-positioned that fabric so that some of it overlapped the reinforcing plate that holds the three studs that mount the door's hinge. Big mistake! The thickness of that fabric was enough to prevent the studs from protruding far enough to accept the nuts.
Note that the boot that goes around the filler and hinge has two holes punched in it that provide access for tightening the three nuts. Why two holes for three, separate nuts? Beats me! The two factory holes are positioned above the two rear nuts, but you're on your own to tighten the front nut. I fixed that by punching a third hole in my boot to allow my socket on an extension to go in. See the last photo.
I hope that my experiences help to save you some time. Be sure to read through this thread for more ideas. Believe me, no matter how long you spend reading this, it will save you time in the end.