well if its a custom, in theory you can attatch it wherever you want. Usually most people choose to attatch it to one shoulder or the other, or both shoulders, or around the waist. Snaps are probably your best bet for attatching it.
I put 4 sew on snaps on my shoulders . In side by my neck and outside by my shoulder . Then put 4 snaps on my cape evenly spaced . This gives me several cape configurations . All on one shoulder ( either one ) or across the back .