Teaching English online is the profession of some English language teachers.
The first requirement is knowing how to teach English. There are many 'English teaching' qualifications available.
The second requirement is a broadband connection with web-cams and voice transmission software. It is impossible to teach English pronunciation, syllable stress, rhythm, intonation, rise and fall, inflection etc. without the student and teacher hearing and seeing each other.
Teaching total beginners online is practically impossible. But online tuition is very useful for more advanced students. They can learn at home at times convenient to themselves. Office workers can learn in their lunch breaks. Professionals and diplomats can learn in the privacy of their own offices.
Online learning can provide opportunites for learning, practice and error correction, thus enhancing their existing knowledge and language abilities.
Learning English involves hearing, reading, speaking and writing. Learning English solely by using online facilities would not provide the sufficient input or output opportunities that are essential to properly developing a student's English language skills.
A person who does not know how to teach English will not be able to learn online how to do it. It would be like learning how to do brain surgery simply by looking at a computer screen or reading a book. We learn practical skills by doing. We learn to teach by seeing someone teach, by receiving tuition, and by teaching others.
All the elements that are essential to acquiring 'language teaching' skills are not possible over the internet. We could learn certain aspects of English grammar and vocabulary online. But that will not equip us to teach others how to speak and write English. It is comparable to trying to teach someone how to play a violin simply by writing to them! Online language learning and online 'learning how to teach English language' or even 'learning online how to teach English langauge online' would face similar degrees of difficulty!