@Murdoughnut - You may not need the FOI written,
but you absolutely have to know the concepts in the FOI. Your DPE may not have the educational credential background that you have but he will absolutely ask you questions from the FOI, especially around principles of learning, educational psychology, defense mechanisms, etc - I highly recommend you study that. Evaluation skills are huge, as well as the concept of the building-block approach to learning. For what it's worth, I have found FOI to be useful in everyday life, I still find it helpful at the airline. It's incredibly useful. My old chief used to refer to the FOI as a manual for
How To Operate A Human. He wasn't wrong.
You will need to know how to draft/build a lesson plan, and more importantly, how to tailor it for the given lesson. A full lesson on XC navigation for a PPL, for example, will be quite different from teaching chandelles to a commercial student. You don't have to follow the FAA suggested format exactly, but you *do* need to make sure you cover the major bits.
Your CFI mentor may have a certain way he wants you to do this. Mine put me in the airplane and had me start talking from pre-flight all the way through the full lesson to shutdown. Talk non-stop. Explain everything I'm doing, and why. It felt silly at first but it got me in the habit of teaching and doing at the same time, and eventually, I stopped spewing words and started actually TEACHING. When you can fly and teach a steep turn, including catching and correcting mistakes and explaining why, while also managing traffic, practice area and student time, you will be onto something.
The ACS for Private and Commercial is your bible. For EACH task, you should be able to provide effective instruction. A syllabus, either school provided or created by you, is the roadmap through that.
Finally - and this is most important - what is job #1, above all others, of the CFI?
It is to care. Everything else - including safe practices, ADM, etc....is all derived from you caring about what happens to your student.