A pole on the ground or Cavaletti. Horses naturally tend to change leads over poles/fences, so if you go over one pole in a figure 8, you can start giving the cue to change leads. So balance right before the pole, new outside leg back, and slight new inside flection. If they don’t change over the pole stop in a straight line and try again. Once they do it on ‘cue’ reward them but don’t over do it that training period. Over time they will understand the cue. Remember to make sure they don’t lose their shoulder during the change cause then they’ll tend to become disunited and unbalanced
It's usually easier to start with a visual for the horse to work off. A pole across the diagonal of the school is commonly used as a point to change lead as it would a) give a visual point, b) usually be placed where a lead change needs to occur anyway and c) makes the horse step higher, which makes changing the lead easier.
As for a cue, it might be best to work with an instructor as there can be a few reasons a flying lead change doesn't work out, and an instructor on the ground is in the best position to be able to tell if it's you, the horse or if you're doing everything right but just not giving a strong enough command.