Forget the BGI. The AGI supercedes it and the exam is barely more difficult. With the AGI you can teach the theory for any FAA certificate except instrument rating. Add the IRI & you can teach the theory for anything.
You will also have to do the principles of instructing exam, whatever it's called.
You can do them all on the same day and get your results immediately on conclusion of each exam. You might need an instructor sign-off to do the exams. Can't remember.
Make an appointment at the most convenient FSDO & get your AGI & IGI issued. It's a separate card from the pilot certificate.
Note: Just about every application for a certificate is registered online. Make sure an instructor gets you enrolled online for this - it saves a lot of mucking around.