You're almost guaranteed to unlock this just by playing the game, but just in case:
Lock 'n' Load folks, it's time for some indiscriminate killin'!
Press
if you need a shotgun or shotgun ammo; it'll be delivered close by.
Now, head to a public spot: A park, motel, graveyard... How appropriate. Unleash hell! When the cops show up, keep shootin'. Just kill everyone, all of the time. Your cheev will pop in no time.
If the cops aren't being as vigilant as you'd like, then try standing by the road and shooting passing vehicles. Let them escape to get the cops called.
Another way is to jack cars and let the drivers live, doing a few in succession until they arrive.
Yet another would be to rob a store, let the owner escape, and wait until the countdown expires to get the cops to show up. Steal the cop car: Instant three or four star wanted level!