As far as i can remember, there are two locations (outside of the Story Levels on Free Play) where you can get this: Bricksburg and Octan Tower, there will be a black and red machine where you can only use with Lord Business. You unlock him in Level 15 and you need to buy him for 1,000,000 Studs. Select Lord Business (or President Business, but Lord Business is better due to some Kragle related achievements where only he can unlock), go to the machine and activate the Leg Machine to unlock the achievement.
If you want to do this on a Story Level on Free Play, go check the Free Play Guides i added on the Story Progression achievements, if you want to do this on a Open World, check these guides