1.Pick your most valuable car (wait for the engine to start)
2.Press Triangle/Y and hold right trigger Quickly (This needs to be timed correctly)
You will know you did this correctly if you see the camera zoom change a bit. Otherwise you aren't timing it correctly.
2.You will spawn outside on foot next to your car. You might be between the garage and the door. That is okay, just drive through the door.
3.At this point, get in and drive away from your garage and call for the car you are in from your mechanic.
4.The real car will appear, and you can then sell or keep the dupe. Be careful not to sell the real one!
IF STORING: At this point drive the dupe directly to your garage, no stops at LSC, should you want to store it.
5.After you store/sell the dupe, reinsure the original, which will lose its insurance, and take it back to your garage.
To do it again, quit to sp and start another invite only session. Do not just find a new session.