What worked for my horse was doing a figure 8 (one canter circle one way and then go the other way and keeps doing this patter that's like 2 circles), and doing change of lead through trot. Then do the same but change of lead through walk (canter to walk then walk to canter) then do from canter to halt and then halt to canter. Then next time ask for the change of lead whilst cantering and they should do a flying change
Do transitions. Right hand Canter, Trot the. Left hand Canter
A way u can do it is if u canter left/right lead toward a pole diagonally and ask to change leads as u are going over the pole
i suggest if you're starting out, cross over a pole on center line, and ask for the lead of the way you're reversing the same way you would ask for the right lead when you pick up the canter.
I've heard of doing a figure 8 over a pole - you ride a circle on the left rein and change the direction to a circle on the right rein over the pole