In reality the best way is to take lessons on a finished horse. Trying to learn even on a finished horse with out help is still hard. It is next to imposible to just on a finished reiner and make them work properly. You see people try all the time and it just does not work well.
If you really want to give reining a try I would say your best bet would be to find someone who knows reining has a finsihed horse and can give you some lessons on that horse and teach you what you need to do to get the horse to work properly.