Its easy mate, i used to do it all the time. Dont use meths or white spirits or you'll be at it for ages!

The best way is to peel off the film and you'll be left with a clear glue-type coating. Get a brand new stanley knife blade and spray the glass with a soapy water mix (washing up liquid is best) and it'll come off easy.

Should take no more than 15 minutes for two front side windows. It wont scratch the glass as long as you keep the blade as flat as you can.