Step 1: Research
First thing I did was to buy the TMNT dvd set. I watched all the movies to refresh my childhood memories of the amazing films. All the movies are amazing but I had to go with the first movie's costumes. The nostalgic value and just awesomeness of the original costumes can't be beat.
I did some research and figured the best approach would be to sculpt the entire costume. Even though I had no experience at sculpting, I wanted something challenging. My first sculpts were laughable. But with the encouraging help from members at the Replica Prop Forum, I was able to hone in on my sculpting and the improvements speak for themselves.