[He'll keep his hand on Eren's diaphragm, watching and feeling the other's breathing technique. Funny, Eren doesn't seem to be a stranger to the breathing technique, just rusty. ]

A little slower...you don't want to pull a muscle you haven't used in awhile.

[He walks him through it a few more times, it becoming clear that there's no fire yet.]

Hmmm...maybe your fire breath hasn't shown up yet. Though I'd probably practice for when it does, few times a day.