First of all I find it much easier reversing with my head out of the window rather than using the mirrors only but that’s me…
Keep an eye on the path the trailer follows when you are pulling off a bay or out of a junction, when reversing the trailer it should be aimed in a similar path.
Always stop with the trailer aimed as best you can to where you want to go, bend the outfit before stopping to reverse, the rig is then set to go in the direction you want it to sometimes without you having to even turn the steering wheel…
Aim the trailer wheels where you want to go, Also, when passing the bay you want to reverse on to look for a marker on the ground like a stain a puddle or a crack in the concrete in line with the bay about 30+ feet out then reverse around your marker and on to the bay. Even after 40 years on artics I used to jump out and drop my spare number plate or a rag on the yard as a marker if there were no lines or other vehicles in an empty yard as a guide.
Practice and practice again, just turning the wheel about half a turn and see what happens as you reverse, when the truck is bent slightly just turn the wheel back to straight and the truck will go round in an arc without you turning the wheel back and too, after all you don’t drive forward off a bay zig zagging the cab so why do it going backwards,
don’t go too slowly when practicing reversing it’s more difficult that way, practice and when you go wrong then stop and go forward and do it again, DON’T try and keep correcting your mistakes, you learn every time you go wrong so the more times you go wrong the more you learn.
Reversing wagon and A frame drag is similar, always set the rig up with the drawbar bent so as it will go in the direction you want it to when you start reversing, never turn the steering wheel more than half a turn when starting to reverse and when you see about a foot of the front of the trailer in your mirror take the lock off and adjust to follow the trailer as you push it, you should never need more than one full turn of the wheel when you get used to it.
Most of all good luck you’ll be there in no time.