Step 5 - Final Sculpts:
Always check if you can attach backpack and shoulder pads- sculpting in hardened green stuff is not always easy task.
Attachment of the chain is definitely the simplest part. Just put it on your character and when you are happy with result use superglue to fix it in place. Be rather generous with glue, the more you add the stronger structure will be.
Of course there are also the skulls. I've sculpted two of them but I do believe using GW ones is much easier and probably will give much better results. One more thing... add a base before glueing in lower part of chain. Finally attach backpack and shoulder pads ... et voila.