|This mission is a setup one for the subtle approach for The Big Score heist.|
You need a Police Transporter in order to pull off the heist, and you will be able to get one at the police station, which is right next to the lot with the Towtruck for Tonya's Missions. You can easily get into the compound with either a police vehicle, or by getting a SUV or other large vehicle and using that as a stepping stone to climb over the fence/wall.
If you climb over the front wall by the gate, there will be an officer to your right. Sneak up on him and knock him out, then steal the vehicle. If you use the other area on the side, there will be two officers to your left, so sneak by them and do the same as above for the lone officer. Take the van back to the hiding place to finish the mission.