Avra learned it accidentally long time ago. I had a lot of trouble with this, because I rewarded it, before having canter
forward on cue
so she would canter only backwards
so I had to make her forget the canter at all, and she did... very well
Now we are learning canter again - from single canter jump. When she does it well, I think that I will get back to canter backwards, and this time I wish to have both canters on cue
If you have something like canter in place on cue (terre a terre or mezair) and the horse has no problem with any backwards movement, you can combine these two. I think that it's not possible for a horse to canter completely in place, so he will always canter a little bit forwards unless you change it to backwards.
And doing "one step at a time" is important of course
This you can practice with easier exercises - like shoulder in one step at a time, or whatever.