You need to use ordinary film too.
Yes, the film is used to protect the screen from breaking. Even if the screen is broken, the broken glass won't fall if the screen is coated with the film.
There's no need to use ordinary film. Ordinary film is easy to be scratched. And screen coated with ordinary of mobile phone is easy to crash if the phone drops to the hard floor. My cell phone dropped from a place that is 1 meter high last time. But there's no cracks on the screen. Only a small gap left on the tempered glss. And no broken glass fell from the screen.
Clean the screen at first. Peel the layer marked by label No. 1 and put the film on screen. Make sure the film is aligned at the screen. Then press the central part of the film. The film will stick to the screen itself. And then peel the label No. 2. If you have used tempered film, there is no need to use ordinary film.
