Have you ever looked? The truth is, there really isn’t one. Perhaps there are some guidelines here, some unstated expectations there, some standards written in a course book, too — but otherwise, you’re pretty much on your own.
Like it or not, that means one big thing: you are the one who writes the rules, and you are the one who decides what kind of teacher you’ll be.
Will you be a teacher that tries to deliver the same unsuccessful English language program they have taught (with modifications) for the past 30 years?
Or will you take advantage of the new opportunities before you (i.e. more English classes, a big push for more English in the classroom) and start work on writing your own manual?
Doing so allows you to discover your own ideas and strengths. It is the way you can bring meaning and purpose to your work. It is the way you can start delivering high-quality education to your students that actually teaches them to speak English.
You can sit around waiting for someone to finally give you the instruction manual. Or you can write it yourself and be better for it.