1. Head to Route 224. It's located south of Hearthome City.
2. Surf across the water to reach Victory Road.
3. Traverse through Victory Road, battling various trainers and wild Pokémon along the way.
4. Reach the top of Victory Road and enter the Pokémon League building.
5. Speak to the receptionist to sign up for the Pokémon League Championship.
6. Face and defeat the four Pokémon League Elite Four members: Aaron, Bertha, Flint, and Lucian.
7. After defeating the Elite Four, challenge the Pokémon League Champion, Cynthia, and defeat her to become the new Champion.
8. Receive your certificate and the prestigious "Champion" title from Cynthia.