This mission will begin at Franklin's house. Get in the van after the cutscene and head for Grove Street. Once you get there, go up to the house and it turns out the guy was trying to sell you some bad drugs. It has now erupted into a big gunfight, so you can either stay as Franklin or switch to Trevor, who has an ability he can use in the battle.
Make sure you use cover to your advantage, as there will be plenty of enemies to kill, including some driving up in cars. You also need to keep an eye out for alleyways and stairwells, as some enemies will be hiding in these. When you get to the end, you finally draw the attention of law enforcement, and will receive a 3-star wanted level.
Follow Lamar to get to the sewage waterway and steal the Seasharks from the people there. Once you board one, follow Lamar and Trevor for a while, until he says to split up. When you do, that is your cue to try and lose the cops. Note that Franklin cannot use his special ability on the Seashark, and it is highly recommended to get onto land and into a car to lose the cops. Once you lose them, the mission will end.
