To start the San Andreas Mercenaries update in GTA Online, players need to follow several key steps involving a new storyline, a major vehicle purchase, and mission access via a special terminal GTA 5 Enhanced Accounts.

 Step 1: Receive Charlie Reed’s Call  
Once logged into GTA Online, you will receive a phone call from Charlie Reed, the leader of the Los Santos Angels mercenary group. This call introduces the new DLC storyline and marks the beginning of your mercenary missions.

If you don’t get the call immediately, you can manually go to Charlie’s location marked with a “C” on the map near Cypress Flats at the Southern Docks of Los Santos. Entering the white marker there will also start the introduction mission.

Step 2: Purchase the Mammoth Avenger and Operations Terminal  
To officially unlock the San Andreas Mercenaries missions, you must buy the Mammoth Avenger, a large aircraft that serves as your mobile base of operations. The Avenger can be purchased from the Warstock Cache & Carry website in-game for around $3.45 million for the base model, with fully upgraded versions costing more.

After purchasing the Avenger, you must also buy the Operations Terminal upgrade for about $1.45 million. This terminal is essential as it allows you to launch all San Andreas Mercenaries missions directly from inside the Avenger. Notably, owning a hangar is no longer required to buy or use the Avenger and its terminal; you can call the Avenger to your location via the Interaction Menu.

Step 3: Accessing the Missions  
Once you have the Avenger and Operations Terminal, call the Avenger through the Interaction Menu under Services > Avenger > Request Avenger. When the Avenger arrives, enter its hold to access the Operations Terminal. Here you will find two mission categories:
1. Project Overthrow: A series of six story missions focusing on taking down Merryweather Security. These missions can be played solo or cooperatively with up to three other players. 
2. LSA Operations: Three private contract missions that act like security contracts or client jobs, offering additional challenges and rewards cheap GTA 5 Modded Accounts.

You can also call Charlie Reed via your in-game phone to launch LSA Operations directly.