I recently got my AGI and IGI. There are no experience requirements and no endorsement required. You just show up at the testing center and take the tests, then take your results to the FSDO. You DO need an endorsement once you get to the FSDO though. I did the Gleim test prep and printed them off from there.
Getting the certificate also requires that you pass the FOI test. I'm not sure whether the FOI you passed as a CFI will still be valid. I believe it lasts two years, but if your CFI is expired, I guess your FOI is probably out of date. Even if it is still valid, you will have to find some way to get a new, embossed copy of your FOI test results to turn in at the FSDO when you go in for your Ground Instructor certificate. Yes, that sounds ridiculous, since your CFI cert proves that you already passed it, but I was warned by the guy at my FSDO that he was keeping my official FOI results and that I would need to obtain a new "original" copy when I come back later for a CFI cert.