STEP ONE:

Have a friend sit in the drivers seat of your car, outside the garage door.

STEP TWO:

Make sure you have a mission request in your phone.

STEP THREE:

Go into your garage. Stand right in front of the middle of the drivers door of the car you want to replace.

STEP FOUR:

Bring up your phone mission, press "X" once to bring up the mission description.

STEP FIVE:

Press "Y" to begin the entrance animation. Immediately press and HOLD LEFT TRIGGER (Don't let go of this until OUTSIDE) Immediately press "A" to confirm mission. Press this combination of buttons, quickly, bu not expressively. ("Y" then "Left trigger" then "X".)

- Remember to hold LEFT TRIGGER until outside.

STEP SIX:

When the mission prompt comes up (this is where you will have to do some adjusting) Most usually just press "B" then "A" immediately to exit. SOMETIMES a pause will be needed before exiting this prompt. (The Background of the mission prompt should remain a blurred version of your garage, if it goes black, the process was not correctly executed)

END RESULTS:

One of two things will happen.

1: You spawn outside next to the sign, if this happens don't bother getting into the car it won't work, try the method again.

2: You will glitch out and end up walking through the garage door (this is important) if you walk out through the garage door, it worked.

Have your friend get of the car. Press "Y", then immediately hold "Left Trigger" to get into the DRIVERS seat. Don't let go of "Left Trigger" until the glitch process is complete and you drive out in your new super car!

TIPS:

After you press "Y" wait a very small amount of time before completing the rest of the process! I've done it twice in a row now with this method. It is not hard once you have it down.