When I started driving a semi,33 years ago, my first job was spotting trailers at a grocery warehouse. For 8 hours every night I moved trailers into and away from loading docks. That was the worst driving job I ever had but the best experience. I learned how to back a trailer.
What I am saying is, the ONLY way to learn how to back a trailer is to practice. Once you get it, you got it.
If you can imagine a line your trailer tires make when pulling out of a spot that is the line you want the tires to follow going into a spot.
I taught my neighbor how to back a TT using empty soda cans as a guide. Just line them up along the path the trailer tires need to follow. Recycle the crushed cans.