Use your scissors to cut your black, plastic garbage bag in half so that you have two flat pieces. Use your scissors to cut one of the pieces to fit the window you are working on. The plastic sheet should be about 1 inch shorter than the window on all sides.
Use your spray bottle to coat the outside -- the side without tint -- with soapy water. Press the plastic garbage bag sheet over the soapy water so it adheres to the window's exterior.
Use your scissors to cut the other half of your black plastic bag to fit the inside of your tinted window. Use your spray bottle to coat the tint sheeting on the inside of your window with ammonia. Press the plastic sheet against the inside of the window so it lays flat.
Allow the sun to heat the black plastic for a about 20 minutes. The sun's rays will heat the glue that holds your tint in place. It should become soft enough to release the tint sheeting from your window.
Remove the garbage bags from the inside and outside of your window. Start at one corner of the tint and use a razor blade to lift it away from the glass. Pull the tint sheeting with your hands until as much of it as possible is removed.
Scrape away any remaining or stuck pieces of tint with your razor blade. Be careful not to scrape or damage your defroster grid in the process.
Use a clean cloth and window cleaner to remove any glue residue or small remnants of the tint sheeting.