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Western saddle fitting

Hi I wonder if anyone can help please. I am in the UK and there are not to many knowledgeable fitters especially where I am.
How do I go about fitting a saddle to a horse, we have a few horses to fit and would like to learn how to do it please as there will be more horse to come.
Any help would be grateful received

http://www.amazon.com/Western-Horses-Pain-Free-Back-Saddle-Fit/dp/1570763895

Mods may want to move this to Western forum.