My advice is dont even think about it. If you are good at hanging wallpaper or cleaning windows, then youve got about 20% of the skills you need to do a proper job on your own motor.
I seriously would not recommend it, unless you are prepared to take all the windows out and do the job indoors, out of the wind and dust etc, and obviously you need to be able to store the car somewhere safe while it has no glass in it, eg in your garage.
You cannot do a proper job with the glass still on the car and if a firm tries to do it like that then theyre cowboys. Proper firms will take the glass out cos they know that it might be a major hassle getting the glass out and then back in again, but its nowhere near as bad as trying to form a thin film of flat tint to a slightly curved piece of glass, inside a car, without getting water and washing up liquid everywhere and bubbles under the film. And if it folds together even by the slightest amount, then that piece is completely ruined.