What do you want to do exactly? I welded and built several tanks, but it is a huge different if you have a leaking tank or you want to change the size of it.
For closing a hole just fill it with water and let as less space with air in it as possible. Than flame this rest and than start working
To cut it is more difficult as water comes out soon ;-) so flame it several times and be carefull.
You also can use gas to fill it (CO) but i like water more as you see it.
NEVER forget it is a dangerous job, be allways carefull
Once a fuel tank of a motorbike explode while i was wellding it. The airbuble was to big and i know it before that it is dangeous.....but it was the only chance to fix it. After exploding i was laying in one edge of the garage and the tank in the opposite edge. It was an experiance i will never forget
Never forget after flaming it that the fuel is still in the material and tent to go out and built new flamable gas. So it is better to flame it more than once and if you have the chance than wait some hours and change water between flaming.