Keep your inside leg on the girth, and take your outer leg behind it, follow this by opening the inside rein, and closing the outside more so your horse will have the chance to fall into the right lead, the direction you're indicating. Lots and lots of practice on picking up your leads, and eventually you should be able to get the right one most of the time, if not all the time.
Inside leg behind the girth and outside leg on the girth, head flexed to the inside, ask with voice, as soon and canter in picked up release your leg pressure and reward with pats and voice "good" or "nice" or whatever you say to reward them
Just really make sure you set your horse up to be able to get the right canter lead by keeping them balanced with the riens and leg, outside back behind the girth and keep your contact light so the horse knows you want them to go forward
Lots of people train their horses differently. From where I am, the most common way is outside leg, inside reign. Your horse has to be balanced going into the canter and balanced enough with their flat work to be able to pick up the correct lead every time.