What is the step-by-step process to become a ground instructor in the US?
You may want to check out this document from the FAA also linked in that document is this document which may shed more light on the matter


  The general qualifications for an Instructor Rating require you have a
  combination of experience, knowledge and skill. Title 14 of the Code
  of Federal Regulations (14 CFR) part 61 requires that pilots must be
  able to read, write, speak, and understand the English language. If
  you cannot meet these requirements of English fluency, an airman
  certificate cannot be issued. For medical reasons, an appropriate
  limitation may be placed on the certificate. 
  
  You must pass the Fundamentals of Instructing Knowledge Test and a Flight or Ground
  Instructor Knowledge Test specific to the instructor rating sought.
  You may take these tests on the same day, and you do not have to take
  them in any particular order. An instructor rating obtained by passing
  the Military Competence Instructor (MCI) exam does not require the
  candidate to take the Fundamentals of Instructing (FOI) exam.
  
  When
  applying for any additional instructor rating, you are not required to
  take the Fundamentals of Instructing Knowledge Test again. Once you
  have acquired a Flight Instructor Certificate, you are eligible to
  give ground instruction required for a Pilot or Instructor Certificate
  or rating, based on the ratings on your Flight Instructor Certificate.
  Because of this, it is not necessary to obtain a separate Ground
  Instructor Certificate since you would already have these privileges.


AskACFI breaks it down nicely here.