An english crupper will attach around your billet straps, like the loops on an english saddle pad would to hold it in place.
Did you get the right style of crupper? If you got the kind with just the metal T part, It slides in the gullet and you just pull back as much as you can until its snug.
Look on this page and you'll see a picture of a crupper adapter and how it is installed... that's how you'd do the metal one.
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