Sorry if its been posted before but yeh
1. Get a car
2. Go to your safehouse in VineWood (not Mad dogs mansion)
3. Drive the car into the backyard where the pool is. Drive against the wall so you dont fall off the edge.
4. do the car flying cheat. Up Down L1 R1 L1 right left L1 Left.
5. Speed into the corner of the wall in the back where the pool is. (not in the pool)
6. You are now in the under world.
7. Pause the map
8. Mark a marker where area 69 is.
9. Fly there
10. Once you get near it, you will be spawned inside.
11. to get out, kill yourself
and thats how you do it