Two lunge lines
Start lungeing in the manège, getting the feel of the two reins. Get this right first as your horse needs you to know what you're doing!
Then have a friend walk at the head so that you can move yourself behind to teach the horse your aids. Practise starting, stopping, turns in the manège until you are absolutely competent before you go out on the road.
Take friend with you the first few times that you go.
This is probably the first time I've seen a long-reining post on this forum; you'll possibly get more responses on the Horse and Hound forum which isUK based. However, I'd be interested in the responses you may get on here.