How do you get the HM flash on Pokemon Leaf Green?

My little brother wants to know.How do you get the HM flash on Pokemon Leaf Green?
From vermillions city, go through diglet tunnel. Once you get out go south past the house into a little gateway building, there should be one of professor oaks aides standing around. If you have 10 pokemon in your pokedex then he will give you HM05 flash.