This will NOT clean a rusty tank, but will get out years of dead fuel debris.
With a post by BD’s, the Krylon King, that makes this MUCH easier.

You need:
A 3 to 5 gallon WHITE pickle bucket. You can but one at ACE or O’Rielly’s…it must be white.
A 2 to 3″ diameter rock
A bendable pencil type magnet, The longer the better, like this or similar:

Gasoline
And, BD’s suggestion, a container of STEEL BB’s. I always thought BB’s were brass or lead, but it turns out you can buy steel ones. They MUST be steel or you will never get them out of your tank. BE SURE they stick to your magnet before putting in your tank.

Put a quart of gas in the tank and the container of BB’s.
Slosh around until your arms are going to fall off.
Remove the petcock.
Drain the fuel into the pickle bucket with the rock tipping it to the side.
Wait 15 minutes.
Put your readers on.
See the debris ring?

Lather, rinse , repeat until there is NO ring of debris.
When you are CERTAIN the tank is spotless inside, repeat the process five more times.

The old gas can be vaporized on concrete.

My KWICK Douche’ Fuel Tank Block-Off Plate Kit helps with this process 🙂