To become a ground instructor, know your stuff, and persuade someone that you do. If you have specialist background knowledge (engineering experience for example) then there are jobs around for ATPL.
I became an ATPL ground instructor immediately after my CPL/IR with no further qualifications, at the company with whom I studied (SFT). The CAA were happy as long as my CGI was, though they monitor all ground instructors at approved schools annually. When SFT went out of business I was self employed, by students that knew I could teach them the material or to whom I had been recommended. Now I work for BCFT, after they made an asset purchase of SFT and asked the old groundschool team back.
I have also taught PPL groundschool - that is not approved, so as long as someone is willing to pay you you can do the work.
Strateandlevel
The advice from my old and wise groundschool instructor on my FI(R) course, a man who became a flight instructor in 1952, about teaching someone in their own aeroplane is "don't". Therefore I suspect you may find a lot of schools resistant.
Note the aircraft has to be brought up to public transport specifications before any instruction can be carried out on it.