Something you put in your post that "piques my interest" is the following:
"It is then that I have real difficulty. I cannot get into the right position to get my foot in the back strap. It all gets a bit "slappy" and out of control. I find it hard to turn and open my shoulders ,thus locking in as I have been told , with the harness on. It seems a bit weird twisting like this. I think I am meant to get the rig back and my body weight forward to get the weight off the back foot allowing movement, but I just luff up and lose control."
I do not understand a couple of things about what you've described here.
Perhaps you can "clarify" what you are trying to do here?
If you move back on the board progressively, keep the board accelerating, you will get your front foot in, and hooked in.
Since you have your weight on the rear foot, directly over the fore and aft centerline so you can "steer" the board (toe pressure takes you downwind....heel pressure steers you upwind) I don't understand why you feel you "out of position" to get into the back strap.
Take your time, let the board accelerate, and simply slide your back foot out and into the back strap. The first few times you miight have to look down to see where your foot is relative to the strap, but very soon "muscle memory" will kick in and you'll know exactly where to slide your foot out and into the rear strap without looking down.
Once you know how to get into both straps, then you sheet the rig in a bit more and rake it back fully until the foot angle of the sail is about parallel with the top of the board and the water.
Now comes the part I REALLy don't understand......
"I find it hard to turn and open my shoulders "
How does one "turn and open one's shoulders".....?
I don't get it.
As the rig comes back and in, yes, you should be facing the centerline of the rig with your shoulders (centerline being a line from the center of the mast to the center of the boom's rear end fitting).
If you have your harness lines truly balanced, this should be almost "natural" as the rig will be sheeted in nicely and raked back and you will be in the footstraps and facing the centerline of the rig with your shoulders aligned pretty much with the CL of the rig.
Now, another bit of mystery.....?
,thus locking in as I have been told , with the harness on.
What does "locking in" mean here?
You may have heard me say you need to "lock your hips and push real hard across the top of the fin with your back foot", but that's a more advanced way to go upwind on fin lift, and has nothing to do with getting your board planing with you hooked in and in the footstraps.
So what is it you are trying to do here, or what is it that others have suggested you need to do?
If you learn to move back smoothly and progressively, all of the "alignments" will pretty much fall into place on their own as you move back, hook in, find the front footstrap, get settled, adjust your course to a beam reach, then work your back foot out and into the rear footstrap.
Once you are hooked in and in both straps, if your harness lines are balanced right, you simply need to lean back (if there's enough power in the rig to support you) straight away from the sail until your arms are fully stretched out.
Hope this helps,