Step 1: Flesh it out.
Think of an idea, and write everything you want about it, all the details as to what it does. It's your idea, pitch it to us. We want to know details, we want to know scenarios, how it will work within the minecraft world. Drawings help as well (but don't feel like you have to draw a picture, not all of us are artists)
While I really like long detaled posts, didn't notch say that he wants twitter length ideas?