Ditching the textbook does not mean that you don't have a curriculum. It just means that you gotta know a few things ahead of time.
What you gotta know before you ditch the textbook.
You gotta know your standards. Or at least be really familiar with them. Textbooks DO sometimes lay out how they relate to the standards, so if you're following the textbook you're getting all the standards... theoretically. If you're going to get rid of the textbook you need to know what your state/national organization expects students to know.
You gotta know what your goals are. This may look different for everyone. Maybe your goals are that students succeed on the end of course assessment. Maybe your goals are that students have mastered some kind of skill. For me, I know that I want my level 1 students to be Novice mid-High according to my national organizations proficiency guidelines. And I know that I want students to be able to talk about themselves and others in a basic yet meaningful way.
You gotta know what your assessments are. You have your goals... How are you going to know when students have met those goals? How are students going to know when they've met those goals? I think that when you get rid of the textbook that people need to be really good backwards planners. I know some people will disagree with that, that's fine. I've seen some teachers go in without a plan every single day (either intentionally or not) and do just fine.
You gotta know how to deal with the unexpected. Much easier said than done. With a textbook, you can meticulously plan every second of your class with ease. When you don't have, what some might call, the crutch of a textbook, what do you do when the lesson is derailed or an activity that you thought was so great falls flat? My suggestion is to have some go-to bail out moves. Maybe you have a Kahoot or Gimkit ready to go *just in case* you need to fill time, or catch your breath. For me, it might be a quick way to turn a listening activity into a writing activity. Even the most foolproof lessons fall flat sometimes, just gotta be prepared.
You gotta know how to find resources. This is probably the toughest part. And I'm sorry, I probably don't have a lot of real advice for teachers who don't teach languages. We essentially have all the knowledge of the world at our fingertips. Even though it's not my favorite thing because I there is so much to dig through, Pinterest has a lot of stuff there, and other teachers who have ditched the textbook have probably been nice enough to share what they like to use. Or Twitter, once you find the right hashtag, like for language teachers #authres to find authentic resources, you will find so many ideas to use in your classroom.
Ditching the textbook isn't easy. I think, though, that it allows teachers to inject more of their personality into their lessons, and allows teachers to teach what they believe is truly necessary for students to succeed.