Instructions
Travel to Cinnabar Island and note that everything is destroyed. Leave the island via Route 20 and head east until you reach the Seafoam Islands.
Go into the gym at Seafoam Island and climb the long staircase directly across from the gym's entrance. Walk up to the trainer directly in front of you and speak with him to initiate a battle. When he is defeated, he will move out of the way.
Follow the path to the west and fight the trainer at the end of the path to make him move. Turn around and head east, past the area where you fought the first trainer, and continue along the path until you get to another trainer. Fight him to make him move out of the way.
Walk along this path to fight three more trainers until you get to Blaine, the Gym Leader. Defeat his Pokemon and you are rewarded with $7080, the Overheat TM and the 15th badge, the Volcano Badge.