Start with finding a shirt that has a detailed pattern or some sort of embellishment.
You'll need scissors, chain, plyers and ribbon.
Cut the detailing out of the shirt and use the plyers to cut the chain to the length you desire. Attach the chain to the top of the detailed pattern.
Attach the ribbon to the end of the chain....tie it into a cute bow :)
Ps. you don't have to attach the ribbon you could use a glasp for the chain....I prefere the ribbon so that I am able to adjust the lenth according to the oufit I'm wearing. Later I glued white felt to the back of the detailed pattern, you can also use interface....this keeps it from being droopy.
Check out Lauren Conrad bib necklace for a video.