Hmm I also have the non-translation problem Bucephalas
But I shall try.
Since your horse can half-pass she obviously understands moving off your leg.
There are some differences however. In the half-pass her head is moving in the direction you are moving...and her shoulders are slightly leading. Now in the leg yield, her head is facing the opposite direction of travel and her body should be parallel to the fence.
You can try it in the walk first, just to get the feel, but doing in the trot can sometimes be better because of the natural rhythm of the trot. Set her up properly with the correct flexion, then try going down the quarterline. You can sit or post, depending on the horse, I've done it either way. I do find sitting the trot easier though.
Then with a bit of inside flexion, ask with your inside leg for her to move gently towards the track. If you are struggling at first, just remember baby steps. After a couple steps done correctly, pat her. Don't let her run, half halt her and get her re-positioned if she starts falling towards the track. But also don't let her start walking either. Try to remain in a steady forward trot. Also don't get too steep at first; rather aim to give her plenty of space to move over at first. Once she gets the hang of it, you can ask for more and more.
Remember to keep her soft throughout the leg-yield. If she gets her head in the air and just runs then you have to go back to basics.
With my horse, it's obvious when it's going right. He starts to get more collected and more organized.
Good luck and I hope this mess helps...maybe...maybe not.