You can put poles towards the end of a diagonal, ice that makes sense. Ask for the change over the pole. Just make sure the pole is at the end of the diagonal bc if it is in the middle the horse might start swapping in front of the jump.
Start by doing walk to canter transitions, once you get those. Go on the diagonal line at the canter and walk at x then pick up the canter again. Once your horse gets that, he will soon enough learn the changes. When you think both of you are ready it's good to use a pole just before the corner and ask with your leg
I start out by using a single pole and canter my horse up to it from one direction and break to trot over it a few steps out, picking the canter up (going the other direction this time, so opposite lead) as soon as I'm on the other side. Repeat this exercise coming from both ways, kind of like a figure eight .
This encourages your horse to swap leads over the pole while he already has his front feet off the ground. Remember to add the cue you want your horse to know as you do this (maybe outside leg inside rein). Keep practicing and be patient, it wont happen over night but if you're consistent, your horse will get the hang of it! :)
You need to ask for it properly! thought my horse can't do it until my friend rode him and she showed me he actually can do it. It was me who didn't ask for it correctly.