Screen protectors aren't as necessary as they used to be, but if you still like to use them on your phone, YouTuber minh8080 has a foolproof method for applying them.
Essentially, you arrange the protector on your screen before removing the plastic back, then apply masking tape to one side of the phone so the protector stays in place. Then lift it up like a hinge, pull off the back, and slowly place it back down. It should stay in place and stick to your phone perfectly straight. Check out the video above to see the hinge method in action.