Instructions
Catch at least 10 types of Pokemon. Find Pokemon throughout Kanto, primarily along the routes between towns. To catch them, attack them until their health meter turns yellow or red, then throw pokeballs at them.
Travel to the Viridian Forest inside Route 2, which is south of Pewter town.
Walk through the forest to the southern gate. Inside the gatehouse you'll find one of Professor Oak's assistants.
Speak with Professor Oak's assistant. If you have 10 types of Pokemon, he'll reward you with Flash.