The Cash Collection Mission can be started from any Stockade as a law enforcement officer. These can be found at any bank all over Los Santos and Blaine County.

To start the Cash Collection mission, get into a Stockade and type /mission or press M and choose the Cash Collection mission. 

You will be given a timer and three pick-up locations. It can help to have a look at the map and figure out a route before taking off, this might save you time in the long run. Note that after you have reached all three locations, you still need to get to the closest bank.

When you arrive at the pick-up location, exit your vehicle and enter the checkpoint in the store. You will be locked in the checkpoint until the pick-up is complete.

After picking up the cash at all three locations, you will be given GPS coordinates to the closest bank. When you drop off the money here, the mission will be completed.

Type /cancel at any time during the mission to cancel it.