Charlotte, like anything else, it's all a matter of learning and experience.
The only time I need or want help backing up to my cargo trailer these days is when I'm forced to do it from the blind side because the kid parked it wrong....
The first few times, you need to do it when you have all the time and the patience in the world...do it when you're in a hurry or grumpy and you'll be cursing like a sailor (no offense to any of our marines...lol.l You'll be in and out of the drivers' seat like a yoyo.
Then you'll learn where to line up the side of the van with the trailer edge and how close you can get. A few more tries and you'll come to within a few inches the first time.
It does take practice, you're not going to be perfect the first time out! If you do, then it was beginners luck....
When I'm close, I leave my drivers' door open, my foot on the step and mentally make a mark on the ground. That way I can judge more accurately what 3" or whatever the distance is.
I also swear by the use of the wheel dolly so I don't have to get the thing within a hair width of dropping on the ball!...