Thorough preparation is the most important prerequisite for a clean result.
First, clean the surface thoroughly. It should be smooth, dry, and free of dust, grease, and other residue.
Then carefully align the film and do not remove the backing paper completely straight away. Instead, work in sections, pressing the film evenly onto the surface with a squeegee as you go.
While doing so, smooth from the area that has already been adhered outward. This allows the film to be applied in a controlled manner and helps reduce air pockets.
You can then carefully trim any excess film along the edges with a sharp utility knife.
For larger surfaces, having a second person can make installation easier.