When you try to peel the film off the glass, the top layer will usually pull right off leaving the second layer behind. This second layer will not peel off, it will just flake and tear like a price tag. This is very important, if you peel the just the top layer, you may as well just use razor blades as there is no way to peel the second layer off without picking at it for a few hours. You MUST try to peel both layers at once. You are more than likely to damage one or more of your defroster lines, when this happens, the cut line will not work anymore.
Some cars have radio antennas mounted alongside the defrosters made of the same material.
Taking it to a tint shop to be done greatly increases the odds of saving the lines, but even a tint shop can not promise to save your lines, sometimes the film is just to far gone. First, clear the window of any obstructions, like the brake light, speaker covers etc. Next cut open the garbage bag so that it will be big enough to cover the entire window. Spray some water on the outside of the back window and the lay plastic bag over the glass. Cut away excess plastic so that it is now roughly the same shape as the glass.