How to attach a harness to a fake noose?

Get a body harness, and make sure the noose rope is slack at the top of the tree, or don't tie it at all on the top of the tree, just swing it over the limb. Under the assumption that it'll be dark and you can't see the top of the tree, of course. Put the body harness under the person's clothes if it's not dark enough outside to hide it.