Am a bit surprised that nowadays you seem not to have to be qualified to instruct.
20 years ago when I became an instructor I took the RTITB course which was pretty stiff - if I remember right it was around six days training which included a driving test for 90 mins (single track country lanes to city centres, I got three minors), a Highway Code test (50 questions) where 79% was a fail but also I had to give a lecture to 30 lads on the rights and wrongs of negotiating roundabouts and T junctions for about 45 mins with the RTITB guy sat at the back. I was also assessed on how I communicated with others, whether I was clear and precise, calm and settling to the nervous.
I passed first shot but only just - I only got just over 80% in the Highway Code questions.
All a bit different now , it seems.