I had cleaned assembled and based the miniature before hand. I used Green Stuff, it has a predictable set time and is stiff enough to span gaps unsupported. After mixing I rolled it out as flat as I could, then put it in between to sheets of plastic. I used a sandwich bag. Then I pressed it between two hard cover books, standing on them in order to get it even flatter. I let the putty set for 20 minutes before removing it from the sandwich bag, then cut it into the size and shape I needed. The putty was stiff enough to hold its shape, yet still workable. Once stuck to the miniature, I used a rubber tipped shaper to smooth folds and curves, and a metal blade tool to create the smaller wrinkles at the top of the cape.
There were a few unwanted wrinkles from the sandwich bag as well as from the shaping. I plan on touching these up with some more putty. Here is the cape from the front: