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easiest way is a utility knife, exacto knife, even one of those retractable box cutters. Dont be afraid to go rough on it, you cant scratch the glass if its a knife w/o any nicks in it. My friend does window tint and he told me that, it surprised me. And with the leftover adhesive from the sticker, you can use a glass cleaner but you'll be out there all day, I'd recommend some acetone or the like.