It should peel off the backing. Be slow & careful, and take your time putting it on ... usually once you stick it to something, pulling it back off to straighten it or smooth it out will destroy them. I used a padded squeegee to apply my KY decals ... lined it up straight with only the left edge peeled away from the backing, and got it started ... then used the squeegee to smooth it on, as I pulled the backing from under it.
Make sure the surface you're putting it on is clean & "warm" ...... I wouldn't try it on a cold or hot surface. And ... it's not just for "windows". Many apply them to bumpers, boats, (boat) windshields, tackleboxes, minner buckets, etc.
... cp