So here is another tutorial not by me that I think is pretty helpful.
I was taking a look online for a Superman logo. My Supergirl cape does not have one because at the time I didn't want one but maybe I do now. I am still not sure.
On my quick search for an embroidered logo, I found this tutorial which I think I might try making if not to put on the cape just as an extra project. I think her cape came out pretty cute!
Jaqs Studio: Childs Superman Cape
This tutorial was created for a child but I am sure it can be made larger. Enjoy!