A spotter job is to spot, and the driver's job is to drive.
The spotter should make sure you do not hit something, everything else is up to you.
Drive backward just like you drive forward, just way slower.
When you are going forward you DO NOT crank the steering wheel, drive forward, slam on the brake, crank your steering wheel as far as it will go the other way, then drive forward again, only to slam on the brake and turn the other way and then get mad.
Go slow, really go even slower.
Use your SIDE mirrors, do not look over your shoulder.
DO NOT LOOK AT THE WHOLE SIDE OF THE TRAILER.
FOCUS, look at where the trailer wheels are going the wheels are the part you are going to park.
Yes, you need to remember where the bumpers are and not hit someone.
Don't oversteer, turn slow and WATCH THE VEHICLE/TRAILER ANGLE CHANGE.
As you turn and move if the ANGLE IS WRONG slowly turn a bit more or a bit less.
If it not going the right way, then turn it the other way slowly while moving
Remember, What you are adjusting is the vehicle/trailer angle.
If you are not moving the angle will never change so do not turn your steering wheel unless you are moving very slowly.
If you look in normal trailer towing mirrors and the trailer is at an angle you will see the SIDE of the trailer in one mirror and not the other one. REMEMBER that turning toward the mirror where you see the trailer will straighten the trailer out.
IF ALL ELSE FAILS Pulling straight forward a little (2-6 feet really) will straighten everything out a bunch.
Use your mirrors,
Here’s something you might not know about backing a trailer: a longer, multi-axle trailer is easier to back up than a shorter, single-axle trailer. Why? Short trailers react to steering input much more quickly, and so it’s easier to get them crossed up when backing.
Backing a trailer has a bit of a learning curve. A good way to build your skills before you get to the campground, stables or launch ramp is to practice in an empty parking lot.
The best way to try to back down a ramp or into a camping spot is, to begin with the vehicle and trailer generally pointed in the direction you want to go. It’s more difficult to back up when you’re starting from an extreme angle. Eventually, you will be able to do that. For now, align the truck and trailer and take it slowly.
The how to:
Once you’re in reverse, the first thing you’ll notice is that getting a trailer to go left means steering to the right. That’s because you’re changing the trailer’s direction with the back of the truck. One neat trick is turning toward the mirror where you see the trailer. Turn gently toward the mirror where you see the trailer, slowly back up, and slowly and it will straighten it up.
So, use your side view mirrors. Adjust your mirrors so you can see the side of your trailer or at least your trailer tires. If you find yourself weaving an S pattern, stop oversteering. Go slow turn slow. To straighten out pull forward a little. Just a little is all that is needed the truck and trailer will straighten right out. This is often much easier than trying to correct huge mistakes while still backing.
When you are just learning to back a trailer, it’s a good idea to use a spotter. Your spotter can tell you if you’re about to hit something or when you are far enough. Even after you become proficient at backing a trailer, a spotter is a good idea in crowded areas and when you have a blind spot while backing. To avoid arguments it should be the drivers' job to drive, and the spotter's job to tell you to STOP before you hit something.
Clearly, no text document can make you proficient at backing a trailer. As mentioned, it takes practice, but it doesn’t take years to learn. After a short time, you should find the feel for it and be able to back your trailer straight in first time every time.