Applying screen protectors, even in these days of guide frames, is quite stressful for me. It’s the dust that appears from nowhere that bugs me the most. In the old days of plastic film it was the dreaded air bubbles.
The knack is to hold the tempered glass protector (application side facing the floor) over the phone and above the guide frame.
Then remove the protecting film on the protector whilst continuing to hold it with the application side facing the floor (this way, dust will not fall and attach itself to the protector via gravity 😉 ) and then remove the protecting film on the phone and slowly lower the glass onto the iPhone screen within the guide frame.
I can do these 100% of the time with zero dust or bubbles - it really is quite easy.