Good film needs clean glass to grip. That's the whole secret. Before we pull out a squeegee, the surface has to be free of old adhesive, wax, and grease. On storefront windows along Groesbeck Highway, winter salt spray leaves a filmy residue on the lower panes, so we budget extra time to degrease those. Skip that step and the film won't seat right, however careful the install.
You personally don't have to scrub anything. We handle the deep clean on site, but all we ask is that you clear the area around the window for full access inside and out, pull off any stickers or old security film you know are coming off, and skip ammonia-based glass cleaner in the days before, since it leaves a residue that fights adhesion. Flag any cracks or chips ahead of time, and plan for the glass to sit undisturbed for a few hours while the film cures.
Vehicle glass is a little different. For a work van or fleet window, wash it a day or two before and skip fresh wax near the edges, since wax works into the seal and causes lifting later. And if it's freezing out, we'll pick a time when the glass isn't ice-covered, since that adds drying time we can't rush. That's really it on your end.