Careful preparation is the most important prerequisite for a clean result.
First, thoroughly clean the surface. It should be smooth, dry, and free of dust, grease, and other residues.
Then carefully align the film and do not remove the backing paper all at once. Instead, work section by section, pressing the film evenly onto the surface with a squeegee as you go.
Smooth from the already applied area outward. This allows you to apply the film in a controlled manner and 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.