|This mission will become available once you have finished Tow Truck, Trash Truck, Boiler Suits, Masks, and gotten a getaway vehicle for the heist. It will be marked with an "H" on the map, in the same parking lot where you met with Steve Haines to begin the setup for the heist.|
You'll being the mission as Michael, so drive the Trashmaster to the initial marker, where you'll switch to Trevor. Look for the armored van and once you find it, you'll be back in control of Michael. You want to park the Trashmaster sideways at the next marker, across both lanes. When you have it in position, stop moving and keep it stationary.
Once the Trashmaster is in position, the game will switch to Franklin and you will have to ram the target vehicle. This will flip it on its side, so get out of the truck and plant a sticky bomb on the rear doors of the van. Blow open the doors and hold the guards by aiming at them, until Michael finishes looting the van.
Of course, there was a silent alarm, so the police have been alerted to your presence, with a lot incoming. Now, they will come from both sides, so you will need a good place to take cover. You will now be able to freely switch between all three characters, with Franklin on the ground, Michael up some stairs, and Trevor taking the high road (with a RPG).
At first, the cops will come from the front, so have Franklin take care of those, but the second wave will begin coming in from behind Franklin, so you might want to switch to Michael to take care of them. Eventually, Franklin will report some snipers appearing on the opposite roof, so switch to Trevor and use a sniper to kill them.
Go back to the ground and continue killing the cops until Trevor flashes again. This time, a helicopter will be barreling down on your position, so you can either snipe the pilot or use the RPG to blow it up, which is definitely the better option of the two. Eventually, the cops will begin thinning out and the number of stars you have will get lower.
When this happens, go to Franklin and get to the Trashmaster. Drive it to the spot you parked the getaway vehicle and before you get in it, use some sticky bombs or something else to destroy the garbage truck. Get away from that scene and the game will finally switch to Michael, who will be delivering something to Devin Weston. The mission will end when you get there.