I first learned when I was very young in the military going through AIT for Heavy Transportation. Don't remember much except you always use the mirrors and NEVER NEVER stick your head out of the window to look behind you while backing.
There are all kinds of tips about where to hold your hands on the wheel so you can remember which way to turn while backing....so on and so forth. I personally don't buy in to any of that. If you have ever watched most professional truck drivers back their rigs you will know what I am saying.
Find an empty CG or parking lot and just practice. Either have your DW on a cell phone spotting you or walkie talkie. Go slow but just keep practicing. It will come quickly.