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.
Sit deep, open the outside rein, shift your weight into your outside stirrup, and push your horse over towards the rail.
You sit deeper, half halt, then switch your leg position so that the leg that is on the same side as the lead you want is at the girth and the other a bit behind
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
Sit up straight, outside leg and inside rein
try to ask when he's in the air