Someone in Mexico who has the independent FM3 handed me an immigration checklist last night showing what's needed, out of the Mexico City HQ anyway.
You need:
-official application form from immigration
-copy of your passport
-an original and a copy of your current visa (likely the tourist visa if you've just arrived)
-letter from an employer (if you are to be employed by someone) stating the nature of the work and pay, on official letterhead of the company
-a letter from the applicant explaining, in Spanish, your qualifications and reason for seeking the visa
-proof of where you live in Mexico, such as a lease, or phone or gas bill
-proof of qualifications - diploma, certificate, etc, etc