Instructions
Defeat all Johto region Gym Leaders, the Elite Four and Lance to gain access to the Kanto region. Defeat the first six gym leaders in the Kanto region before you head to Seafoam Islands to find Blaine.
Make sure that one of your Pokemon knows the move Surf. Head to Pallet town in the southwest corner of the contiguous land mass on your map. Surf south from Pallet Town towards the small island in the Southwest corner. Battle and defeat any trainers which appear until you reach Cinnabar Island.
Take Route 20 along the coast of the island. Encounter and battle the trainers as they appear. Land on Seafoam Islands. Take the middle ladder to enter the Cinnabar Gym. This gym uses a different puzzle than earlier generations so you must defeat several trainers to find Blaine.
Battle the trainer directly in front of you. Defeat this trainer and he will move back, allowing you to proceed. Follow the path to the right and battle the next trainer. Keep going past this trainer to yet another gym trainer further towards the north end of the gym. Continue to employ this strategy to get past the next trainer and the final trainer, a "Super Nerd," to reach Blaine.