I'll try doing the video, but in the interest of a quick response I'lll provide what I've learned to self-don and doff the suit:
What I've found to be most difficult is getting the zipper started at each end. That is where the tricks come handy.
1. Before putting the arms in the sleeves, I pre-start the zipper by closing it about 4". With the zipper closed 4-6", I can still get mr armed and head into the suit . For someone larger, it may need to be less, but key is having the zipper started.
2. Try to keep the suit and zipper tight by rolling the shoulders forwar, bending the left elbow and arm, while reaching behind the head and grasping the zipper with the right hand.
3. Once started in either direction, you just have to flex the arm and shoulder behind the zipper to tighten the suit and cause it to pull against the zipper pull held in the other hand.
It's more about using the arm and shoulder not pulling the zipper to help pull the suit against the zipper than it is about being flexible. If you can raise your arm straight up, bend your elbow behind your head and touch the top of the opposite shoulder, flexibility shouldn't be an issue.