14 CFR § 61.419

Receive a logbook endorsement from the authorized instructor who trained you on the applicable areas of operation specified in § 61.409 certifying you have met the aeronautical knowledge and flight proficiency requirements for the additional category and class flight instructor privilege you seek;

Except as provided in paragraph (e) of this section, successfully complete a proficiency check from an authorized instructor, other than the instructor who trained you, consisting of the tasks in the appropriate areas of operation contained in the applicable Practical Test Standards or Airman Certification Standards (incorporated by reference, see § 61.14) as listed in appendix A of this part for the additional category and class flight instructor privilege you seek;

Complete an application for those privileges on a form and in a manner acceptable to the FAA and present this application to the authorized instructor who conducted the proficiency check specified in paragraph (b) of this section; and

Receive a logbook endorsement from the instructor who conducted the proficiency check specified in paragraph (b) of this section certifying you are proficient in the areas of operation and authorized for the additional category and class flight instructor privilege.

If you are seeking to add an airplane single-engine land or sea or a rotorcraft-helicopter with simplified flight controls designation privilege to your flight instructor certificate, successfully accomplish a practical test for that category and class privilege as specified in § 61.405.