What cue do you use on the ground? Your cue on the ground should be: stand at her side, near her shoulder, crop in hand. Tap her side slightly behind the girth with your crop and angle her head slightly towards you. She should sidepass neatly away from you in response to the crop on her side. Teaching it this way makes it much easier to translate under saddle.
The cue under saddle is as follows: Using your right leg, nudge her behind the girth, the same spot she takes her crop cues from. Angle her head slightly to the right, using a little left rein to keep her from over turning her head. If she has a habit of going forwards, keep a little pressure on both reins to prevent that. If she has a habit of going back, use more leg and less rein until you find a balance. Your leg left should be completely off her side, "opening the door" for her to move right. Sit balanced in the saddle, straight and not leaning in either direction. Keep your aids quiet and CLEAR.
Reverse cues to go the other way, naturally.