To start a Routine Patrol Mission visit the checkpoint at the Police Department of the current city, type /mission and select the first option. The mission will start. As soon as the mission starts you'll get the location you'll have to get to and your time will start running, so make sure you put a car ready before you start the mission. Respond to every of the 5 locations before the time runs out. Type /cancel at any time during a the mission to cancel it.