Natural Horsemanship is a belief that a horse is an animal with a brain of its own and as such deserves respect and kindly treatment from humans.
A trainer/owner/rider picks up the knowledge as they spend time with horses.
As a start I'd say buy any books you can find written by a member of the Dorrance family - especially the old man of the clan - Tom. - who was himself only semi literate but he knew his subject - the horse and the horse rider.
The books are hard reading but if you eventually come to understand what Tom was meaning to say, then you are on your way to understanding the tenets of NH.
Another writer to read is Vladimir Littauer - who at one time coached the US team for the horse Olympics. He was as a young man an officer in the White Russian cavalry who emmigrated to the US and who became a world renowned trainer of both horse and rider. Buy all his books - if you can get them off the internet.
First mission for you might be:
for the horse to come to the paddock gate and whinny at your arrival.
for the horse to doff its head so as to make it easy for you to fit the head collar; then for the horse to follow you at the shoulder on a loose lead line to the tacking up area, there to wait patiently as you start the process of grooming.
It might take you months to achieve, and maybe you can occasionally cheat by giving tidbits, but it is the foundation of your relationship with the animal. It is the start of the bonding process.
Not everyone believes in NH but there again not everyone has the patience, knowledge, time and temperament to achieve a bond with their chosen horse.
NH is all about non verbal communication and mutual trust.