Vinewood Souvenirs - Mark is a Strangers and Freaks mission for Trevor. This is one of the more annoying side missions in the game due to the layout of the golf course (good luck finding the entrance) and lack of cover.

Upon entering the Golf Course, you will be instructed to treal Mark Fostenburg's golf club. He is the red blip on your radar. Approaching Mark will have his body guards, and the cops, hunting you down, so kill the guards quick. Killing Mark will not halt mission progress, so don't worry about the cross-fire. Steal the club and lose the cops, after that is done, Trevor will call Nigel and the mission will be complete.

Tips:

Snipe Mark first, then worry about the bodyguards. He will make a run for it as soon as you're spotted.
Do NOT steal the golf cart to escape. It's slow and handles like crap. You can pick up the club from the back of the golf cart and then make your getaway. The easiest way to make your getaway is to jump into a water hazard and submerge yourself. No need to leave the course or find a vehicle.