How do you teach Zero English?

I am not a teacher either. But as I see it there are basically two ways to approach this. One would involve using a common language that both the teacher and the student speak as a crutch during the first stage. You would do that while you cover the basics of the target language (pronouns, basic vocabulary, sentence structure, and grammatical quirks). Then you would gradually start to shift away from that second language and encourage students to communicate and formulate questions in English.

The other approach would be acquisition-based learning. That means that you would start speaking and explaining everything in English only from day 1. This method only works if the students are getting the gist of what you are saying, so it’s paramount that you use all resources available to support comprehension. That means using lots of hand gestures, mimicking, and pantomime as well as pictures, video, and every visual support you can think of. I would also start with the most basic words like pronouns and verbs, and gradually start to add new words and structures. The key here is letting the students slowly get a feel of the language and hopefully get to the point where they can start expressing themselves in English even if they use incorrect grammar. When they get to that point you will have a base to work from and you can start refining their form.

Whatever the case may be, I would go easy on corrections. They undermine the student’s self-confidence, especially in the first stages. What’s most important is comprehension, so as long as you can understand what they are saying I would avoid correcting them. Instead, try to lead by example and maximize exposure through lots of conversation, text, audios, and videos. After a while, exposure to the language will make them click and they will start to self-correct.

As Leo pointed out, pronunciation is key when learning English. The reason for that is that the way English words are spelled has little to do with how you pronounce them (e.g. though, tough, thorough, though). That’s why it would be a good idea to introduce the International Phonetic Alphabet (IPA) at some point and teach the pronunciation of all English vowels (12-20 depending on how you count them). That would allow students to look up the correct pronunciation of new words in a dictionary whenever in doubt.