It's all down to preparation and cleanliness. I've only done it once and was pretty pleased with the result; used some film off ebay, here's the (very) basics
The glass must be completely cleaned and then a bit cleaner. Cut your film to the right size, newspaper templates are the easiest way, and put a bit of sticky tape on both sides to help you get the backing off it. Spray the glass with a mix of water and a small ammount of washing up liquid, which makes it possible to slide the film on the glass to some extent. Seperate the tint from its backing and smooth it on, sqeegee the liquid out towards the edges to get it to stick, but be careful not to scratch it with the squeegee...
For the best job, remove the glass so youcan work on it flat and get right up to the edges.
There's lots of "how to" guides out there on t'internet, mostly American, but they explain it a lot better than I can.
Have a go yourself, and if it goes well you can feel chuffed with youself, if it goes wrong just take it off again and no one's any the wiser......
Andi