The Paleto Score is the fourth heist mission you complete during the main campaign. This heist is planned out of the necessity to acquire funds for another mission being planned by Steve Haines which involves stealing a chemical weapon from the IAA at the Humane Labs and Research. The achievement will pop after the heist is completed.
The only setup mission for this is to intercept a military convoy and steal the hardware they are transporting. You'll start the mission by driving to the bank in Paleto Bay to commence the heist. After collecting the money from the vault Micheal, Trevor, and the gunman will put on heavy body armor that you had stolen from the military and confront the police. Trevor will be equipped with a minigun which you'll be able to have a lot of fun with. You'll fight your way through the cops and then the military until you get to a railway and board a train to escape the area ending the mission.