Right at the start of the episode, you'll be recreating the first time travel scene from Back to the Future, in the parking lot at the mall. After Einstein time travels in the DeLorean, you'll get two dialogue choices, one after the other. You need to pick the following two;
"You disintegrated Einstein!"
"You built a time machine out of a DeLorean?"
Once the conversation is over and you gain control of Marty, the achievement will unlock!