Near the start of the game you will complete a mission for Mario that unlocks your rebel drops. During this mission you will drop off a helicopter (this is unmissable). As soon as the mission is over, turn left and you will find a jetty nearby. Grab a boat and bring it around to the garage, there is a spot for boats, but only some boats can be able to be used so you may have to trail and error. Finally, just behind the garage is a car-park, grab any car and bring it to the garage and done. Achievement will pop.