After finishing Three's Company, switch to Trevor and start doing anything until you get a phone call from someone. Accept the call and it will begin this mission once you switch to Michael. The guy you rescued before mentions someone living in Rockford Hills, so Michael and Dave are told to head over there.
When you get to the house, you find it's the wrong guy, so you will switch to Trevor and have to torture the guy. For now, pick the pliers and follow the instructions to pull out a tooth. If you decide to use a different method, keep a check on K's heart rate and health with the monitor. He should mention that he knows the guy who was named, saying he lives in Chumash.
That's your cue as Michael to get back in the car and drive to Chumash. When you get there, Dave calls Steve and asks to get a description of the guy. Back in control of Trevor, choose another method of torture (preferably a different one) and get some info out of the guy. Once you get it, you go back to Michael, who's controlling a sniper rifle. Look through it at the house to find a party going on, so you ask to get more info.
This will happen a few more times, where you don't get enough information from the guy being tortured, so you will need to torture him more to get new info. In the end, you're looking for a left-handed guy that chain smokes Redwoods, and has a bushy beard. Look through the scope and zoom all the way in, fixating on the person until you hear Michael saying he's taking the shot, so do it.
Once the guy is dead, you switch back to Trevor and drive the tortured guy to the airport. After dropping him off, the mission will end.