I'm writing a memoir (for school) from a first-person perspective of someone who lived in the Soviet Union and looking for more information on the selection process for the Soviet secret police, specifically the Joint State Political Directorate.
Originally, the CHEKA was originally drawn from Petrograd Bolshevik members. As it grew into the 1920's Felix Dzerzhinksy, the man Lenin put in charge of the CHEKA after its initial head, Moses Uritski, was shot and killed, recruited from members of the Bolshevik faction he knew to be trustworthy and not squeamish. Basically, it was an invitation only club, one could not simply join, one was recruited.
EDIT: I forgot to reference your original question, the CHEKA was reorganized in the early 1920's into the Joint State Political Administration (OGPU), basically changing the nameplates on the office doors, Iron Felix was still running the show.
_Ronald Hingley, "The Russian Secret Service: Muscovite, Imperial Russian and Soviet Political Security Operations, 1565-1970".