Instructions
Defeat the Elite Four in the Johto region to gain access to the Kanto region. Take the train to the Kanto region.
Surf to the Seafoam Islands found in the middle of Route 20. Enter the cave and continue down through the sets of stairs into the basement levels. Bring along a Pokemon that knows the move Strength. Using Strength outside of a Pokemon Battle lets the monster move boulders out of the way so that you can continue.
Navigate the dungeon's twisting tunnels and boulder-moving Strength puzzles to the fourth basement level. Walk up to Articuno's ice platform and save the game.
Start the Pokemon battle with Articuno by facing the blue bird and pressing the "A" button. Lower Articuno's health meter until its remaining health is small and red. Inflict status-altering ailments on Articuno such as Sleep, Freeze or Paralysis to make it easier to capture. Bring many Ultra Balls, Heavy Balls and Timer Balls to catch Articuno, as it can break out of Pokeballs with ease.
Reset the game if you accidentally knock out Articuno. Avoid moves that cause Confusion, Poison or Burns; those status ailments may accidentally finish off Articuno before you can catch it.