After finishing Grass Roots - Franklin, you will see two big green circles on your map. Go to the one on the east side of the map and find a spot where a vehicle will be highlighted by a blue circle. This is the one with the drugs that Barry needs to be picked up. Get in the vehicle and drive it to Barry's apartment, but you will likely get a two-star wanted level when you get onto the road.
To complicate things further, you only have a few minutes to get the drugs to Barry, or else you will fail the mission. That means you will have to lose the cops within those four minutes and then bring the goods to Barry. Find a good hiding spot and wait for the stars to disappear, then deliver the package to the apartment. Exit the truck and walk away to finish the mission.