It is easier for the horse when it is 'in the air' so start by practicing over a cavaletti using the tips mentioned in the other answers. Then move on to just using a ground pole so the horse will still have a bit more air than usual on the stride. When this is all going very well, start practicing without any rails.
Very hard to explain it in writing, u need to be on the horse much easier this way lol. Just quick tip u need a horse that knows how to do it to do your training on him/her.
Good luck! It's very useful skill to learn