When the mustang has no prior knowledge of the saddle, this process is easy. In this clip Maurin introduces the saddle to Douglas, who has a history of reactivity. Douglas takes it all in stride.
Mustangs that have been handled in the holding facility or failed adoptions often come with hidden histories of traumatic exposure to saddles, ropes, and cowboy attitudes. The resulting handicap can be overcome by careful re-exposure. See the MagicMustangTamer.com for help on this issue.
Good luck with your training. Be sure to visit our training site if you need more help.