Straight up hardcore grammar? You need a big-ass education to teach that.
The grammar associated with "today's" lesson? You need a grammar manual to background yourself, and then you need a sense of where the students are at in their own usage, and then you keep it simple: a structure you can write on the board, and a usage, the one you've practiced today. Monkey's can do that. Just don't let them think on their feet and make up rules they haven't researched.
Like: present continuous plus time stamp for future plans with some current reality.
"Date? No, sorry--I'm washing my hair tonight."
Teacher: What's Mary doing tonight?
Student: She will wash her hair.
Teacher: Yeah, she will--but what's she DOING tonight?
Student: She's going to wash her hair?
Teacher: Yeah, she's going to, but what's she DOING tonight?
Student: Going to washing her hair?
Teacher: No, what's she DOING tonight?
Student: Why not "will"?
Teacher: We're not doing "will" today.
There probably was a better way to do the exercise, but they had eight other -ings to get through and they were going to have used will if they weren't corrected. I'll go out on a way simplistic limb and say none of us would have our grasp of grammar if we weren't in effect, but much less systematically, corrected in the same way. (And then you do fluency exercises and smile if they use the introduced grammar.)
We don't need to know so much about grammar as we need to know about graduated levels of sophistication of usage. You need a big ass education if you're going to do that properly, too, but you can pretend one lesson at a time if you remember the rule about not making up rules.