In addition to all the great advice so far...One of the big things to be aware of, specially when reversing, is that the trailer cannot turn on a whim unless you have been preparing for that turn.
If there is an obstacle you aiming for (say a gate) and you are already turning full lock, and you miss it, you will need to move quite a bit back to restart the maneuver. There's no shame in this, and it's easier to restart from a better spot than to try to move the trailer sideways with short distances... it's just not going to happen.
Think of it similar to pushing a wheelbarrow, you push the wheelbarrow right, to make it turn left, and you plan ahead of time to make sure you are aiming with enough runway to adjust your position and the wheelbarrow, and if you missed you target, you need to back out enough to be able to re-establish the direction of the wheelbarrow.
EXCEPT...
You are on a big car, and your wheelbarrow is quite large, and does not pivot on a single wheel, so movements and correction take longer to take.
Regarding using human assistance...
If you have a spotter that is not familiar with how trailers work, you need to manage their assistance a lot, They may think you can turn much later...Sometimes I think my spotter is much more versed and it will catch up with me,.. everything is fine, but just at the end, you are almost there, just 2 feel to the left now....NOPE, impossible, need to reset big time. This guy tows a lot..what was he thinking? My fault for trusting, smile and nod. only happens once in a long time.
For very complex reverse maneuvers I have my volunteer "drive", but only at my very direct instructions at very slow speeds while I'm spotting. Half turn this way, stop, I will turn it for them and get them to reverse a few more feet, etc. This works the very best in my experience in very complex situations, because it's impossible to see all while driving, but if you don't understand how it works you can't spot well either, so I am spotter and "remote control" driver by asking them very very clearly and specifically what to do with the wheel. There is a long driveway with some curbs and a tree clearance I need to go in a few times a year. This works best for that situation for me.
Keep in mind this is where some marriages fail and crumble. Take responsibility as the leader of the activity for the any failures. If it's not working, re-assess. Realize it takes practice to get perfect. Don't get mad, and don't scream unless needed to save life or limb.
BEST OF LUCK!