Step one: Remove filler bolt with a 3/8" ratchet (without any socket attached). Better than a ratchet would be a "solid" ratchet, i.e. one without that fragile mechanism. I myself use a ratchet, which got damaged a long time ago and I welded the mechanism solid. To that ratchet I attach a long pipe to increase the lever-arm. The picture above is actually from when I tightened the filler bolt again, but to open it, you obviously have to turn it anti-clockwise.