How do Charizard to learn fly in Pokemon Red?

Charizard cannot learn Fly in Pokemon Red.

Here's why:

* Fly is a TM (Technical Machine): In Pokemon Red, Fly is not a move that Charizard can learn naturally. Instead, it's a move that needs to be learned by using a TM.

* TM availability: The Fly TM is not available in Pokemon Red. You can only get it in Pokemon Blue and Yellow.

To summarize: You can't get Charizard to learn Fly in Pokemon Red. You would need to play Pokemon Blue or Yellow to access the Fly TM.