on a mk3 golf GTI (with the spoiler ofc) i would spray a light mist of water on the OUTSIDE of the window.. then stick the tin to the outside of the window and trim the bottom edge, using the glass in the window as a guide to run your knife round, must be an incredible sharp knife or else it may crease the tint..., then clean the inside of the window, with meths if available to get rid of all dust and static, then spray the window with water and a tiny drop of washing up liquid (not hand soap or any kind of skin cleaning products as it leaves a residue behind, hense why some people get ripples in their tint!)
then peel the backing paper off (if you struggle use 2 pieces of tape to grip each side while you peel them apart), then scrap it from the bottom upwards.. once you start to get to the top curve of the window then put a slight slit in the top corner, but leaving about 15mm away from the glass, then slowly scrape the tint at 450 to the rest of the window, if you see a crease forming follow the last procedure with the slit, if you leave the 15mm you will find that it should tear itsealf into place until it touches the window..
then leave it on the car for ATLEAST 1 hour on a warm day, or 2 hours on a cold one... then trim where required..
i will have to do a tutorial for the forum one day if enough people are interested?
some people find it easier doing it otherways, this i find the easiest method how ever...