Instructions
Travel to the Viridian Forest, south of Pewter City.
Walk through the forest into tall grass until you encounter a Pikachu. There is a six-percent chance of encountering a Pikachu during the morning and daytime. At night, there is a five-percent chance.
Engage Pikachu in battle and lower its hit points into the critical red zone. Use any status-changing moves to make Pikachu easier to catch.
Throw a Poke Ball at Pikachu to catch it. If it breaks free, continue throwing Poke Balls until you capture it.