If the windows are flat it's fairly easy... First clean with soap and water, Then use a top grade glass cleaner (Stoner INVISI-GLASS is the one my installer used... Strangely enough I already had a can of that in the van [very highly recommended])
Clean BOTH sides (inside and outside) of the window
now, slap some soapy water on the OUTSIDE and apply the film... Use a HAIR DRYER or hot air gun to heat-shrink to fit (That's the trick the instructions don't tell you) use it carefully, but the stuff will "heat shrink.
Trim as needed, soap up the inside, transfer the film to the inside and squeege out all the air bubbles
Clean and dry, move on to the next window, same procedure
Sounds easy... Trust me. It is, if you do it a dozen times a day, every day, for a year or more
It's not so easy for folks like me (DIY) I tried, failed, paid for a pro