The easiest way to do this is if you use your phone to order a tank.
I don’t think it matters which island you are on but pick an area that has a nice bit of road that loops.
Then start running over CARS. THIS IS THE IMPORTANT BIT. Don’t try and run over anything else or you will get stuck. You can get hooked onto the tank barrels as well so stay clear of them when going for this.
The best thing to run over are the police squad cars. They will drive right at you and basically want you to run them over.
If you’ve avoided getting stuck, you shouldn’t really need a new tank but if you do, you don’t need to order a new one. Just wait for a tank to show up and car-jack it.
The main thing to do is ‘JUST KEEP MOVING’.
Jump into the new tank and repeat.
Be careful of the cops on foot. They are the only thing that will try and pull you out of the tank you are in. Just run them down. :)