A couple of points to note:
1. The film is notoriously difficult to apply yourself without getting creases or bubbles in it. That's why there are specialist firms that do the work.
2. Remember that the film works by letting people on the 'bright' side see through it but blocking the vision of people on the 'dark' side. That means that people on the outside can't look in during daytime but the effect is reversed at night, meaning that people in a brightly-lit room can't see out but they themselves can be seen clearly (as if in a fish tank) by people outside.