Barry can be found at the same spot where Michael found him, in Loren Square/Strawberry Avenue, sitting at a table. Once Trevor takes a hit of the weed, something similar to Michael will happen, only Trevor will be battling clowns. These clowns are a lot more dangerous, but they only need one hit before they disappear.
After the first wave or two, some clown cars will start appearing. You need to destroy these, as they will continually spawn more clowns, so go after these when they show up. At the end, there will be a single clown that will split into two when killed, with them splitting farther to a certain point. Keep killing them until they're all gone and Trevor returns to normal, which will bring the mission to an end.