Exams...
You can take the FAA exams at many testing centers in the US. I've always used CATS via the local community college - something like $129/test. AOPA members save ten dollars.
You will have to take The FOI test as well as any BGI, IGI, and AGI tests you want. No instructor endorsement required.
Take the exam results to the FSDO and eventually get your instructor certificate in the mail.
I have BGI and IGI.
Hope this helps.