If you want to you can use this guide or read my descriptive text.
Credit: Batman Arkham Videos
To get I'm Batman you should watch the intro and afterward you take control of Bruce Wayne. You lead him towards Line A, and then get thrown into an elevator. You learn to be able to counter and strike by defeating the 5 inmates. You pick up Jack Ryder, although you get hit in the back with a baseball bat and finally get stomped by Penguin. After you wake up, you break Penguin's arm and then free yourself from handcuffs after countering a few people. Soon as you take them out, (if you want, you can take Penguin out,) you go and parkour up to contact Alfred. You talk to him to give you an immediate drop on the Ace Chemicals building, you go up the building and finally put on the batsuit to get this steam achievement