Do not pull the backing off and try to apply the sticker.  It will end up crooked and wrinkled.

Open the YouTube page and watch the instructional video.  It's for decals but the steps are the same for bumper stickers.  If you don't have a smoothing tool a small book works just as well.  You will also need spray cleaner, paper towels, masking tape, and scissors.  Our stickers can be applied to bumpers, windows, tailgates, and body panels. 
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​https://www.youtube.com/watch?v=bRkURz3GrDQ