We start by us getting out and agreeing on exactly where we want the trailer to end up. We both look at low branches, etc. I look for obstacles near the roadway, overhang issues, etc.
My DW stands there with her arms crossed. Sometimes she's texting on the phone. I don't move if I don't see her and she is acknowledging I exist. She'll stop me if I'm going to hit something, but that's about it. She rarely uses the same hand signals, no matter how many times I tell her it's really important.
I always stop, get out and look if I'm unsure - which I do every time, since I'm never sure if DW sees everything. The only hand signal I trust of hers is the last few feet when she signals me to stop. You should see us when I hook up the bumper pull, she'll guide me back then tell me I missed, try again (it's easier to jump out and look 5x as I back up).
Last year I went camping with my brother for the first time. He doesn't own a trailer nor tow a trailer. He jumped out and started guiding me into a camp spot. Perfect. What a difference between he and my DW, his hand signals made sense.
When backing up into a site on the blind side (passenger side), it helps to have an electric mirror on that side which you can adjust as you backup.
I always get out and look as I backup. Sometimes only 1 time, usually more often. Don't be afraid to pull forward and adjust or start over.