Simply load up Mission 10: Violence and Timing. Continue until you are hanging from the helicopter about to cut the rope. Once you drop onto the vehicle below you, HIJACK it, this is your first vehicle. There are 4 more vehicles that you need to HIJACK, the only exception is the Civilian cars which are on the side of the road parked. For these you must pull up besides them, and the prompt will be to DRIVE it. The other 3 vehicles you need are:
1. The little Ute / 4x4
2. The medium sized vehicle with the Armored enemy sticking his head out
3. The big truck with all the crates in the back
Remember to try kill anyone shooting at you as you approach them and jump over to HIJACK their vehicle.
I created this video to help:
Hope this helps.