Any tips on teaching a mare how to counter canter? She's never done it before.

Thank you for the suggestions everyone!
slight bend to the outside and outside leg on the girth, inside leg behind the girth. You should ask on the long side and square up your short sides until you can hold it better. At first you will probably be more bent to the outside so you can hold it better. As you practice, the aim is to have the horse more straight in the movement.

Also, the horse has to be able to do a nice collected canter and be working off his hind before you should even try this. Just running around on the wrong lead isn’t counter cantering, that’s just being on the wrong lead.
Get her to canter on one lead then change the rein but keep her flexed to the outside. If she changes do it again and keep doing it until she gets it