I think I can help…
So it took me quite a few years to learn to feel my diagonals and now I feel like I can teach anyone. I have a five step process.
1st: have her sit the trot
2nd: tell her to feel how her hips move slightly to the inside and slightly to the outside with the stride. You, as an instructor, can see if she is able to “feel” it. If she just bounces up and down with the trot or if she is able to move her seat forward and in/out with the trot. I have found the word, “shimmy” helps a lot of people. Its a shimmy to the inside & a shimmy to the outside.
3rd: When she feels that tell her to say “now” or “up” when she feels her hips going to the inside, but DO NOT raise with the “now” or up. She is just saying it, no posting.
4th: have her walk and then start the process over until she is reliably saying it properly. most of my students get this right away. You need some student (esp. younger students) to walk in between cause they will cheat and just keep track of what is correct in the head.
5th: then have her start the process and say up five times correctly and on the sixth time have her stand in her stirrups when she says up. Most students will actually stand right after they say “up” and sit when they say up because of the second it take for the message to go from the butt to the their brain to their legs. And this is why they get it wrong. They feel it, but then they rise incorrectly and so you tell them they are wrong. They are feeling right, just executing it a split second late.
Have her practice that using the verbal up and make sure she raises when she says up. Like I said, most students will say “up” then rise, “up” then rise.
After that it just takes practice. I seriously have been able to teach all of my students within 10-15 minutes. It took me year to figure out that I was feeling it right and just coming up late. Hope this makes sense and if it does, hope it helps.