Watching the video is helpful for this step. I also put the phone on a heating pad for a couple of minutes to soften the adhesive.
This operation went fine in my case, but I was concerned with what seemed like a lot of necessary force to get the supplied suction cup to adhere to the screen - I didn’t want to crack the glass. After a few unsuccessful tries, I did finally moisten the suction cup and it stuck solidly. I was also surprised that some careful prying was needed to get the screen to release.