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.