on the lunge, on a 20m circle. Sometimes exclusively with sidereins, sometimes without.
A horse MUST be able to balance on a circle *without* the rider before they can balance with. They must be relaxed in the back, and stepping through with the hind.
It doesn't hurt that the voice cues are firmly ingrained from this work, so that "walk" means WALK and you don't have to worry so much about the down transition. And canter (whatever cue you use, I use a sharp "shht" backed up with the lunge whip in the beginning) means canter now, not run into it.
The horse must be balanced BEFORE cantering, and you should only canter as many steps as STAY balanced. Careening around like a motorbike only builds WRONG muscle, not good muscle.
Early on, when the young horse offers it--say uphill, or on a straight away, I take what they offer, never punishing for it, always praising, and *trying* to bring them back before they loose balance.
Some horses, particularly those who are very short or very long in the back, will be well established in lateral work and even beginning collection (i.e. *sit* ) before having a decent canter under a rider. My stallion worked piaffe before canter.

We *still* have days where the canter just isn't there--he's not sitting enough--he's particularly short coupled--and we only do four or five strides on a long side, not even an entire circle. But WHEN it's there, it's THERE. So it is worth the patience and the wait. As he becomes stronger in it, it will be equally up to the level of the other work. Right now, it is holding us back from showing--it isn't strong enuogh yet to ask for the figures in the level of tests where the rest of the work is.
Sometimes the RIDER needs to canter quite a bit on a horse they trust, who is balanced, to get back into the feel of it, and to work the abs so they are a more balanced, positive burden for the horse. Riding greenbeans exclusively sets you in very bad habits, and so the downward spiral begins. If you can get the feeling of a good, uphill, "fun" canter in your mind and muscles, it will be much easier both to let go, and to shape the young horse.