try asking the horse when on a corner to canter and sit in to him from walk the horse should automatically go go into the canter mode as you are preparing for the corner shorten your reins sit with both legs on the horse once mastered you can try down the back straight from walk to canter

the reason i say try on a corner as most horses when 1st taught the canter its easier for them to learn and easier for you as a rider from sitting trot to ask on a corner for the horse to learn to pick up pace - all your doing is omitting the trot part