Here's a breakdown of the steps:
1. Obtain the HM05 (Flash): You'll need this HM to see in the dark areas of the gym. You can get it from the old man at the top of the tower in the north-west corner of Cinnabar Island.
2. Challenge Blaine: Go to the gym and battle your way through the trainers.
3. Fight Blaine: Once you reach the top of the gym, you'll face Blaine in a fierce battle. He uses powerful Fire-type Pokémon like Arcanine and Rapidash.
4. Defeat Blaine: After winning the battle, Blaine will award you the Volcano Badge and TM46 (Thunderbolt). You've now unlocked the Cinnabar Island Gym!
Important Note: Be sure to prepare a team of Pokémon with good moves against Fire-type Pokémon, such as Water-type, Ground-type, or Rock-type moves.