This mission is available when you have finished Paleto Score Setup, which will place a mission icon on the map every hour or so, in-game. The convoy is constantly moving in a set direction each time it appears, which is random. It will consist of some escorts and a truck with the equipment you need for the heist.
What you want to do is create a roadblock of some kind to stop them, if you can. Remember that the equipment cannot be destroyed, so make sure you don't damage the truck at all, but the escort vehicles are open. You can also open fire on any of the vehicles, which will likely make them return fire. Once the convoy is stopped, kill all of the enemies, steal the truck and then bring it back to the meth lab to finish.