Solo Method:First, you will need to find a Comet which is a small white hatchback. To do so, I recommend running laps on the highway, or finding one in the streets, although it doesn't spawn very often. Once you do, stand in front of it to make it stop, then jump upwards and you should land on top of it. Once you land on top of it, it will begin to move and you will get a "Car surfing" prompt one your screen which you should accept. You will now need to balance your character as long as possible until the car hopefully explodes. There is no way to make this more likely, so there is a small luck factor in getting the car to explode.
Co-op Method:Have two people run around the map together until they find a Comet. One of the players should quickly hijack it, and the other player will jump on the roof. Once this is set-up, the driver will begin to drive and the player on top will get a "Car surfing" prompt which they should accept. Once that happens, the driver should look for a small lava pit or damage the car by hitting things for it to explode. After this is successful, trade roles for both people to get it.