Here are the steps to get the eighth badge:
1. Complete the Indigo Plateau: After beating the Elite Four and becoming the Champion, you will have access to Kanto.
2. Travel to Blackthorn City: It's located in the northwestern part of Kanto.
3. Challenge Clair: She is waiting for you at the Blackthorn City Gym.
4. Defeat Clair: Her team consists of Dragon-type Pokemon, so bring Pokemon that are strong against them (like Ice, Rock, and Fairy-types).
5. Receive the Rising Badge: After defeating Clair, you will be awarded the Rising Badge, your eighth Gym Badge.
Remember that you can also get the eighth badge before becoming Champion, but it's a bit more difficult, as you will be battling stronger trainers.