Yeppers, Saddlebag's got it right!
As you are leading her, be watching for any signs that she is thinking about nipping. Her ears may start to twitch forward and back, or she may wrinkle the corner of her nose/mouth up, or some other little sign. As SOON as you see her thinking about it, bend the arm nearest to her in a sharp angle and aim a quick solid poke with with your elbow towards her muzzle. She will have made her move and will be swinging her nose towards you just as you are raising your elbow up and the point of your elbow will thump her sharply in the jaw or nose.
It will seem to her that she
ran into a hard painful object, instead of her learning the game of "bite and evade the slap"...because they are much quicker moving away than we are at slapping!
Drop your arm back down, don't say anything, don't react in any other way, just keep walking and leading her like nothing happened. Keep an eye peeled and do this EVERY time she even thinks about nipping and she will soon give it up.
Good luck with her!