Very informative thread, some notes:
The criminal records check can only be done, for US citizens, through the FBI: http://www.fbi.gov/about-us/cjis/background-checks
This document must then be sent to the State Department for an apostille: http://www.state.gov/m/a/auth/
Once returned to you in Spain, it must be translated by an official “traductor jur?dico.” Keep in mind the FBI documents must not be more than 3 months old when submitted to the Registro Civil at the start of the nationality petition.
The birth certificates follow a similar procedure, but on a state level. One requests the certificate, then sends it to the relevant state apostille agency, usually also named ‘State Department.’ For example: http://www.dos.ny.gov/corps/apostille.html
This, once returned to you in Spain also needs to be translated officially. Once you have those two documents the rest is easy, grab two Spanish people who know about your life and head to the Registro Civil.
I did it last june. I had two interviews, one from a nice woman from the Ministerio de Justicia who came to my place, and one at the Polic?a Nacional. Both were very similar - where do you work, quick summary of basic facts, etc etc.
Now I’m just waiting. It’s important to keep Justicia updated with any address changes as communication is via post only. There is a website where you can check the progress of your petition: http://www.mjusticia.gob.es/cs/Satellite/es/1200666550200/Tramite_C/1215326363758/Detalle.html