This achievement is pretty simple. It just requires you to play as Tofuboy. On the character select screen (what appears right after you select a level), simply type "petaphile" on your keyboard, select Meat Boy, and the achievement will unlock.
If the character select screen doesn't pop up, then that means you haven't unlocked another character yet. Just unlock Commander video (the easiest character to unlock) before you do this.
If you have any ideas on how I can improve this solution, I'd love to hear them! There may be something that isn't here that should be, or an easier way of doing this. I'm always open to suggestions. If you notice any typos or poorly phrased sentences, please let me know.