A liberal amount of water or soapy water should be used behind the film to make installation easier and help in removing air bubbles. Excess water will evaporate in time, generally 24-48 hours.
All you need is a drop or 2 of dish soap in a spray bottle, too much soap may cause a problem.
Window film installation may look imperfect until water fully evaporates. If installed properly no large air bubbles should remain. You may want to protect the window film surface with a piece of freezer paper or the backing paper when installing since when the window film is first received it will be more delicate until it has time to fully cure.
Removing as much of the water possible is very important especially when using the non-adhesive window film since too much water left behind the film may cause the non-adhesive window film to release from the window.
Unpack the film and allow the film to lay flat for 24 hours.