Ok, so lets get started. First of all, we need a reference picture. I found this one on our trusty Google Images http://just-imagine-dolls.com/art/robert_tonner/tonner_hp_HERMIONE_HOG_l.jpg:
Then I went to this website to get the pattern: http://www.drakenfjord.org/how2clok.pdf. The makes a proper cloak with sleeves if you like and not just a half circle one. Really good! Most of the cloaks I make use this as a base pattern.
Then I bought some really stiff black material, cotton but stronger as it would have to live through a party of youngsters. The material has a name but I cant think of it at the moment... For the lining I chose some simple shiny lining in burgundy. I still had a clasp lying around so that bit was free :) This is the end result:
It overlaps a tiny bit in the front and I have no idea why but everything was already hemmed etc so I just left it.
The patch I found on eBay. There is this one seller that has all the movie badges etc. My steampunk pins also came from there. It really looked great on the cloak :)
All in all I was very pleased with it, especially since I had zero measurements to work with. All I got was 'just like you but a bit smaller' LOL. Yes sis, next time I need a bit more! Hihi. But it fit and she was happy so that was good.
So technically, if you have the materials already, try making this cloak. It really doesnt have to cost much at all! Good luck :)