Get a BMX and ride it to the most northern safe house in the Mulholland area near the Vinewood sign. If you are interested in buying it, the cost is $120,000. Notice that the building is in a "L" shape. Go to the inner corner of the "L", next to the pool. Ride into the walls and you will fall through one of them. You will go into a glitched area. Perform lots of rotations before you land back near the safe house to get an extreme stunt bonus.