Do not attack it physically with a blade, this is how you end up cutting the defrost lines.
Attack it chemically.
The key is the use of ammonia (Formula 409, some blends of Windex, straight ammonia, whatever else contains ammonia). Spraying it on the film and applying the garbage bag allows it to sit and permeate the film (instead of evaporate off) and react with the glue. I'm guessing the heat aids in the ammonia to pass through the film, also with the flexibility of the film as you pull it away..
If you want professionals, just google "how to remove window tint". There are plenty of youtube window professionals outlining the procedure.
If the tint has been applied to the edge of the glass, maybe cut a small slit in the corner to allow your ammonia-based-fluid-of-choice to penetrate. Some say to turn on your defroster to help add heat to the film. If the film does not remove cleanly, apply ammonia-based-fluid-of-choice to the sticky side as you pull away.
I've removed tint on a rear window that was blacked out for Texas and Central Valley heat. If you're apprehensive, try the side window first.
