Faa Cfi
There are two written test you have to take. One is derived from the FAA "Basic Helicopter Handbook" (I think that is the correct title) and the other is " Fundamentals of Instruction".
The first sort of speaks for itself and the second covers building blocks of learning, Learning curves and so on. Gets into the shrink side of teaching people. Boring, but worthwhile reading.
Otherwise, "memorizing" the answers from a study guide may be your best bet as you already have instructing experience.
You will be surprised to see that the Feds still teach running take offs, and approaches to the ground for operating in conditions where the reserve power is not enough to hover!!??
Have fun.