All begins with anatomy.
So google human anatomy or go to pick up any anatomy for artist (or doctors) book. Vintage anatomy book for medical use has really cool pictures in them.
You can either do freehand drawing or trace suitable bodypart from the skeleton to shrink plastic.

You can also be very creative and try black shrink plastic. Then just cut out the shape and use white marker to create the bones. Take time to do the drawing and do not be afraid. Use variety of different lines, dots, hashes, small cirles to crate texture and depth to your drawing.
Then cut out the drawing and leave 1 or 2 small "extra" circles, where to make holes with hole puncher for the chain to be attached.

Bake the shrink plastic as instructed. There is quite many different brands and quality of shink plastic. Only way to know which one is the best, is to read user comments on different boards or simply just try out different brands.

Once you plastic bones are ready, it is time to attach the chain. You can use many different kind of chains, ribbons, laces to create your jewellery. Pearls, stones, glitters gives very good finish to Your jewel.

and for the end, just a simple succestion. Keep you work place nice, neat and well organized. It is much more fun to work in place you actually have space to do something. You also find all equipments more easily.

Please, do have lots of fun with shrinking plastic and anatomy books!