First you need to know which way the dog turns. Usually we start this by having a dog retrieve dead balls (if they already fetch) or throw treats.
Then, we usually do a few sessions of very ungracefully luring the turn by having a person stand in the middle of the lane with one foot on the bottom edge of the box and luring the dog around the person. This is mainly just to get a dog used to the idea of touching/hitting the box with their feet. At first the person is pressing on the box so that the box won't "pop" like it does when the dog actually hits it, since some dogs are afraid of that at first.
Then, we use an assortment of props to basically force the dog to launch onto the box and then turn quickly and come off the box quickly. We have all different sizes of jump boards, gates, cones.... some people will build little "forts" around the box. Most of our team dogs always have at least one prop and the only time there are no props are running in tournaments (like in gymnastics, we always have extra safety mats in practice even when we can do the skills). I change my props each session depending on what we're working on in that session (launching onto the box, angle off the box, snap offs, strides onto and off of the box, etc). At home I work my dogs on a push board a few times a week. Not too often or for too long since it's very repetitive but also at home I'm constantly switching props.
My dogs both figured out the concept of the turn pretty fast but it's really about building the muscle memory and using props so that the dog becomes consistent with a good turn. Pan for example will get sloppy and start sliding into the box real quickly if props are removed too often in training. Nikon's technique seems to hold up better but I still always use props in training, mostly because I can and it helps promote a safe turn all the time.
I use a pushboard to train turns at home. These are some training vids that give an idea. The first vid shows me introducing Nikon to his board with me luring. At first my board is elevated and my props are really exaggerating the turn to force the dog to do it correctly. My dogs did not have any problems transitioning from a elevated pushboard to a regular box in training (I use props there too, just setup differently).