We have been out to a local, large parking lot where she has been learning how to drive around cones and backup.
Other than experience, that's about as good as it gets.
I also use 16' long lumber and make a "parking place" in the middle of the parking lot. Have her back the trailer into that space without running over the lumber. Then have her make 90° turns while backing into the space - simulating backing into a driveway from the street. Also have her make 90° turns while going forward without running over the lumber but getting the trailer tires within a few inches of the corner of the "parking space". It will probably be frustrating for her to practice turning while going forward or backwards until she gets enough experience doing it that she will know what to expect the trailer to do when she turns. Backing while turning is the tough part to learn, but the only way to learn is to practice, practice, practice without having buildings or trees or other vehicles nearby in case she still needs more practice.
Darling Wife has got the going forward down pat by now. She rarely clips a curb while making a turn going forward. But she still needs more practice backing the trailer into a space or driveway.