First off. If you don't have experience with Greenstuff the result might not always be to your liking. Maybe you can look out for bits to help you. Like Dark Angel legs/bodys or 3rd party ones like those:
http://www.anvilindustry.co.uk/index.php?route=product/product&path=77_96&product_id=133
Only read along if you want to try it with
GS.
Do you want to create a full robe with only feets showing? If this is the case the first step would be to fill in the gap between the legs. Its best to use some cheap filler. I used aluminium foil for example. Fill it in and fix it in place with
GS on top. But just enough to create a solid base for the folds to be sculpted on. Let it cure.
Then mix together enough
GS so that cou can create the body of the cloak. It might be wise to pin it before that so that you have something to hold them model without touching the
GS.
The trick now is to sculpt the folds into the
GS. This is the actuall tricky part. It helps al lot if you have a reference - either pictures or a model. And soft tipped tools like clay shaper are really helpfull to get a smooth finish on your folds. And you should plan your folds slightly ahed, make them at least somewhat realistic looking. Maybe look at cloth folds tutorials.
This is a full custom sculpted body without a marine underneath. But I used the same approach. First build a basic shape and than add the fold onto it. The folds are not fully realistic but it looks decent. And don't feel shy to touch up your folds afterwards. As you can see I did the same. The lower part of the robe is a bit lighter and you can actually see where I fixed up the ends of the folds.