This is the first achievement you will likely get in this DLC and in order to start it you will need to travel to
T-Bone Junction.
Once there your first mission will be to see Scooter, he will give you a mission called 'Boost The Monster' which tasks you with finding 3 car parts for his new vehicle. Once collected he will give you another mission called 'Greasemonkey' which has you build the Monster. Place the 3 parts and upload the specs and the achievement is yours.