Step 1: Choose who the banana telephone is for
Choose who the banana telephone is for may seem like an obvious, almost subconscious, step, but sometimes it's not. It's lots of fun to make a really awesome project, but it does tend to help if you know who you're making it for.
For example, don't make this project, then realize that the only person you haven't bought a present for is your seven-year-old sister. I'm sure she'll like it, but really, it's like giving a Barbie doll to your dad.
I made the original prototype for my just-married sister and her husband. It's always useful to have a telephone in the house, and (as I told my mum) they're already crazy with love; now they can look crazy, too!