Not sure of any defined training as I'm sure 90% of us just did it and learned on the fly. I started with a 24'5th and had white knuckle grip the 1st 10 miles of the 30 mile trip home. The start was a simple turn out of the dealer lot and then a merge to the highway. Glad I got used to it on the highway as I was able to feel it out for lane placement and where things were in the mirror. 10 miles later I was relaxing and it was a easy drive the rest of the way. 2 turns in town and I quickly learned about turning wide as I jumped curb the 1st time I turned right at a traffic light. Backing into the back yard was a real trial and error exercise but again you will quickly learn what works and what don't. I kept trading up in trailer size and weight in a few foot increments and almost no difference once you learn on the first few outings. 36' and 16,800# trailer now but had 34', 33', 27 ' rigs on the way up.
(Spent and lost $$ on each trade so better to spend extra time deciding what you want and need right at the start).
Randu