Instructions
Keep Golbat in your traveling party after it evolves from Zubat or after you catch it in one of the caves.
Press the "X" button and then select the "Pokémon" command to view the six Pokémon in your traveling party.
Highlight Golbat and select the "Switch" command to place Golbat in the first position of your Pokémon team. From now on, Golbat will always fight first in battles. Every battle that doesn't result in Golbat fainting will increase its friendship with you.
Travel to the National Park, which is north of Goldenrod City. Collect the Poké Ball on the ground in the northeast corner of the National Park to find a Soothe Bell.
Press the "X" button, select the "Pokémon" command and highlight Golbat. Select the "Item" command to give Golbat the Soothe Bell to hold. This item increases a Pokémon's friendship with its trainer.
Feed Golbat any stat-enhancing items you pick up during your journey, such as Protein, Carbos, Calcium or Zinc. A Pokémon grows more fond of a trainer that boosts its fighting ability.
Enter the Goldenrod Tunnel that runs under the Pokémon Center in Goldenrod City, and then visit the salon kiosk. Let the stylist give Golbat a makeover to boost its friendship with you.
Visit the girl in the house that is to the north of the Goldenrod Department Store. Press "A" to talk to her; she will tell you how well your Golbat likes you.
Check back with the girl in Goldenrod City every couple of days to see how your friendship with Golbat is growing. When it's strong enough, Golbat will automatically evolve into Crobat.