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 some open fields.
Get your wanted level up by firing off some rounds and wait for a tank to show up. It might take a minute before one shows up, just keep moving around. They will send in cops and then the SWAT vans and jeeps. Once a tank shows up just stay out of its range (a red circle will show where it will fire next). If you keep moving you can probably destroy about 7/8 tanks before you need to get a new one. I let my tank get down to 2 green bars of health before getting a new one, since after that it slows down quite a lot and is harder to avoid the tanks fire.
Now here’s the good part. You don’t need to order a new one. Just wait for the next tank to show up and car-jack it.
The main thing to do is ‘JUST KEEP MOVING’.
Jump into the new tank and 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. :)