I suggest starting from the ground.....doing a lot of groundwork and working your way up. That way you cover any basics that your horses original training may have missed, as well as create a really great bond with your horse as well as get him to trust and respect you, which is what natural horsemanship is all about.
I just bought a Green horse that was broken the traditional "cowboy" way and I am starting him over using Natural Horsemanship..... I have always followed these methods as part of my own nature, but even in asking questions and reading books (Pat Parelli and Julie Goodnight are my favorites) I'm learning even more things to make me a better horse person.... I think it is really kind of sad that sometimes people on here (and you know who you are) choose to reply rudely or belittle someone who is asking for more knowledge in the interest of making life more enjoyable for their horse.
The only stupid questions are the ones left unasked :P
Clinton anderson has videos on treating ill mannered and disrepsectful horses as do all the big names who get problem horses but he has a good " colt starting" video or 3 videos of "gaining respect" with some mild ground exercises that you can do with your horse.
It is fun to watch the improvement such as stepping over, moving backwards and sending over poles, jumps and or ditches without touching and free lunging work
We used Anderson as a baseof information and moved up to our own way of ground exercises based on what most of our horses loved to do.
I would also do a parelli horsenality chart on your horse to see or provide a marker of what can or might need to be improved and how the horse would most likely improve from the ground exercises. I believe it is free on their website or at least the chart.