Ok so in Episode 10 of Attack on Titan 8:17 to 12:47:
Eren: I'm... going to go away
Armin: Where to? How?
Eren: For now, anywhere will do. Then I have to get over the wall and find my old basement. Before that, I'll need to become a Titan again.
Armin: You can do that?
Eren: I don't know how myself. But I think I can. Just like you can't explain how your arms move. Earlier I just focused on blocking the cannon fire. That's why my body had no real functionality or durability. This time I'll try something stronger. The 15-meter class form I used to kick the shit out of all those Titans!
Mikasa: Eren your nose