To enable an online police chase, you'll first have to have people in your session. You can check this by either looking at your mini map on the bottom left of your screen and seeing how many people there are next to the little car icon. You'll need at least 3 down there.
Now you'll be looking for a cop car/ambulance or fire truck. Hit them repeatedly until it triggers a countdown saying "The Police are targetting you!". If 2 or more people accept a phonecall asking them for assistance, then you'll have yourself an online cop chase.
Now all you have to do is run... Well... Drive. Make sure you don't get caught up or else they might hit you with an EMP which will disable any power from your car.
You'll have a time limit of 2 and a half minutes to evade the cops, not getting arrested in this time will count as 1 escape. Just 29 to go!