I wrote something similar somewhere else here, but...
Get a piece or two of baking/oven/wax paper, then mix your gs, and put it between two layers of said paper.
Now you flatten it with a rolling pin/glass/bottle until it is thin enough and let it cure a bit (depending on temperature and ambient moisture, around 10-15'), so that you will not leave fingerprints and it will not stick everywhere.
Now cut an arc of circumference, anywhere from 60° to 180° wide, with a radius about the height of the model, shoulders down. Cut away a small circle where the neck will be.
Helping yourself with pens, pencils, toothpicks, cotton swab or any small circular objects, give it a wavy appearance; I'd suggest a slight curve trasversal, near the neck (to have the cape "blown" backward) and some radial waves - bigger near the middle of the cape - for a billowing effect. You may also make the cape lower on one side, or improvise, to achieve a natural look.
When you are happy with the shape, let it cure completely before attaching it to the model.
Mmmm I could be clearer than this... But I trust you'll get what I meant