The longer you have done something, the more likely you are to underestimate what you know.
Your course idea may be hiding inside questions people already ask you.
Think about the last six months:
What have colleagues asked you to explain?
What mistakes do you regularly help people correct?
What can you now do easily that once felt confusing?
What result have you helped somebody else achieve?
Your expertise does not need to be revolutionary.
It needs to help the right person take the next step.
What question do people repeatedly come to you with?