Recently we spoke to a modern foreign languages school teacher who planned to teach English at home to foreign students. She had just completed a Cambridge Certificate in Teaching English to Speakers of Other Languages (CELTA) course.
She found it very useful and learnt a lot about how English works as a language and how to describe English grammar in standard Teaching English as a Foreign Language (TEFL) ways.
Many secondary and primary teachers have some experience of helping students, often recent immigrants or whose parents are , who need help and assistance with their English, particularly vocabulary, reading and writing. This makes sense as they need to read and write to function effectively at school. However foreign students who come to Britain on short courses often have different priorities and want to focus on their speaking skills. Also they have often learnt about English grammar at school using terminology that is not usually used in British schools eg present perfect, 2nd conditional, phrasal verbs etc.
Cambridge CELTA or Trinity CertTESOL 4-week level 5 courses can provide you with an excellent foundation for teaching English as a foreign language even if you are a qualified MFL school teacher. They cover, amongst other things, speaking skills, pronunciation, vocabulary, grammar and methodology. Many universities, colleges and some language schools run these courses. They will be of even greater benefit to teachers who do not have a language background.
CELTA courses are now available partly online. The blended courses combine online teaching and face-to-face teaching practice.