Instructions
Obtain a police car. This is the most difficult step because the only way to obtain a police car is to steal it. Police cars can be found on nearly every street, usually on patrol. Just run up to the driver's side and use the hijack command to take control. Nearly all parked police cars are locked, so you must find a police car in use.
Lose your two-star wanted level. Stealing a police car will earn you two wanted stars, which makes you the center of attention for all other police vehicles and officers in the area. To erase your wanted level, simply drive outside the flashing circle on your map. Once outside the circle, avoid the sight of any police for several seconds until the wanted stars disappear from your screen.
Activate the police computer and search for criminals. Now that your wanted level is cleared, you can browse the police car's criminal database and perform police missions. Bring up the database by pressing the "R1" button on the PS3 controller. Select the "Most Wanted" category and you will find a listing of criminals, along with their profiles. Pick any criminal and his or her location will be marked on your map by a waypoint.
Perform the police mission. Drive to the designated waypoint and eliminate the criminal by any means necessary. Once the mission is complete, you will be rewarded with police funds and are free to choose another police mission from the car's database.