I don't know any tutorials off hand.
However you can extrapolate from some reference.
Here's a big playlist of flags.The basic theory is to pass waves thru the object.
How complex or simple you want to represent that is up to you.
It would be helpful for you to post a flag you are trying to work with -----> a ref and an attempt of your own.
A flag can come in different shapes, sizes, constraints, materials, etc.

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And knowing that it is, we seek what it is... ~ Aristotle, Posterior Analytics, Chapter 1