Once you've given the resin a minute or so to fully penetrate the glass chip, remove the suction cup tool from the windshield. Quickly place a section of the clear finishing film over the still-moist repair area. Use the razor blade to carefully press the resin toward the edges of the film. You aren't trying to get it out of there fully; you just want it to be as thin and evenly spread as possible.
Best Windshield Crack Repair Kit
Some people like to add a little tape to the film to be sure it won't slide around while the resin is setting, especially if it's a windy day.