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How to become a horse eventer
Go to useventing.com and find a few certified eventing trainers in your area.
When you find one you like, start taking regular weekly lessons with them.
After a while of taking lessons, go ahead and lease a horse from your trainer.
This will allow you to experience ownership without jumping into it unprepared. After a couple years of lessons and leasing, start shopping for an event horse with your trainer.