Fixing a dent on a motorcycle gas tank is as easy. Here are the steps (watch the included video if you don't want to read):
1. Sand the bare metal or paint. Bondo can go over the top of paint or primer and will hold, but on bare metal is best. use a grit somewhere in the 80-200 range to get enough grain in the surface for the Bondo/filler to mechanically adhere.
2. Mix the Bondo per the instructions on the can and using a plastic spreader smooth the substance over the dent. Fill the entire dent and then a bit more so you have something to sand to perfection later. If the dent is huge you really need to pop it out as much as possible so you aren't using too much Bondo in one hole, which could crack or pop out later.
3. Let it dry. 20 minutes to a few hours depending on thickness. If you try to sand it and it gums up the paper it is not cured.
4. Sand starting with about 200 grit and keep going with higher grits up to 1200-2000 grit until it is as smooth as the surrounding materials that are primer ready. You really want to not feel or see any edge of the repair or it will show up in the paint. The edge of the Bondo needs very careful feathering so that it doesn't look like a hard line, but rather like a gradient of Bondo merging with metal.
5. For a deeper dent you may need to repeat the entire process to build up enough material to have a perfectly contoured end result.
6. Bottom line is that it is super easy to do and you can pretty much just go for it without knowing much. All the art is in the sanding and if you are careful, patient, and go slow, you will have good results.
I've included a video is a video of me putting one layer on a tank that eventually needed two layers to get it perfect.