First you have to pass selection for pilot training in the Armed Forces. Then you have to complete sufficient flying to be selected for a course at the Central Flying School, then when you graduate you become a B2 QFI - there is NO SUCH THING as a civil 'Q'FI!
However, if you merely wish to upgrade your civil FI(R) rating to FI (there being no such thing as an AFI any more), you must achieve 100 hours of flight instruction and have supervised at least 25 solo student flights and have secured the recommendation of a nominated supervising FI.