A friend of mine does tinting professionally, the important part most people don't do is to form the film on the outside of the glass first. Use a knife to trim to the correct shape (make sure you know which side is the sticky, and have that side facing outwards on the outside of the window (so the removable backing is the closest layer to you - think about it and treble check), then use a heat gun to gently heat the film and chase out bubbles/wrinkles. Once you have done this make sure the inside of the window is properly clean, use fairy liquid to degrease. Then remove the backing, apply lots of soapy water to the adhesive side and the glass, then start chasing bubbles out from inside. If you skimp 10% on prep, the result will look 60% worse.