I have got some solar film top apply to a couple of internal windows at Church (actually to obscure them when we build some noticeboards behind them - if you see what I mean).
The glass merchant said to cut it a bit large, wash the window well will soapy water and apply with it still wet - using a squeegee to smooth it out and squeeze out the water. Then trim the edges with a sharp blade.
Any experts like to give extra advice/guidance?