This mission begins at Franklin's new apartment ("F" on the map) and can be started only by Franklin. It appears that Lamar has been set up by Stretch and is in danger, so Tanisha asks you to go and save him. It's going to be a long ride up to the sawmill in Paleto Bay, so make sure you get a speedy ride to get there.
When you finally get there, you will need to set up your assault positions for both Trevor and Franklin, as Michael will go up to the top of a nearby hill with a sniper rifle. If you want a lot of action, put both characters to the right of the starting position. Once you're ready, signal with Franklin and the chaos will begin.
Start by taking down all of the Ballas in the initial area, then start heading up the hill. You can either keep shooting with normal weapons, or blow up the vehicles with explosives, as they will use it for cover. You should see one of the characters shouting about some snipers, so switch to Michael to take them out. One is on the roof, while the other is on a pathway on the right.
Continue fighting until you get to Lamar, killing the enemy next to him, then start heading back to the vehicles. There will be a lot of new enemies that will now appear, so push through them all, switching to the other characters if you want. When you get to the vehicles and all of the enemies are gone, get inside one as Franklin and begin the LONG drive back to Lamar's house to finish the mission.