First, you must dent your metal tank. Don't hit the tank cover because it will break (like mine)! The best way is to hit around the cover.
When you dent it, you must dent a plastic cover. To do it, you must use a heater (the best is 3kW or more). I had 2 heaters (both 1,5kW). You must heat it from inside (thanks to that the paint from the cover wouldn't fall of).
When the cover is hot you must fast get it on bike and pull hard down on the tank. USE GLOVES - it's very hot!!! Do it quickly, because it's getting cold very fast.
When you do it, than you can heat it on some small area and do the form as you want.
Firstly I have melt my cover and push it down. Than I was heating a smal areas and do the good form of it. At the end I have done it smooth by using some special plastic 'gel' with fiber glass. Cover after melting is geting very hard, so plastic + fiber glass is OK
Sorry for pics - I was doing them by phone