How do you get the flash technique in Pokemon Red?

You can't get the Flash technique in Pokémon Red.

The Flash technique is only available in Pokémon Gold, Silver, and Crystal. It's learned by the Pokémon Mareep and is used to light up dark caves.

In Pokémon Red, you can use the Strength technique to move boulders, and the Cut technique to cut down trees. Both of these techniques are useful for navigating through caves and other areas.