To start the mission, head to Michael's house with Franklin. After a lengthy scene, you'll be in control of Trevor. Head to the marked vehicle and go to Grapeseed. When you get there, some stuff will happen and a gunfight ensues. Make sure you use cover, as there will be plenty of The Lost coming after you. You come equipped with a pistol and shotgun, so definitely use the latter.
Keep killing them and eventually a truck will roll your way. Shoot it once or twice to set it on fire (or make it explode), then get back to work on The Lost. After some more time, two will drive up on motorcycles, so make sure you make the kiss the dirt before moving on. When you see them running away, that's your cue to get back to your truck. Drive back to Ron's place to finish the mission.