Instructions
Catch a Golbat using a Luxury Ball. The Luxury Ball will increase the Happiness of the Pokemon inside it at an increased rate. You could also catch a Zubat with the Luxury Ball and evolve it into Golbat.
Give Golbat a "Soothe Bell" item to hold. This item increases the holder's Happiness at a faster rate.
Add Golbat to your traveling party of six Pokemon so that wherever you walk, Golbat will be with you. Every time you walk 256 steps, the Pokemon in your party gain Happiness points.
Level up Golbat by gaining experience points, either through battles or using the "Exp. Share" item. Leveling up will boost Golbat's Happiness every time.
Feed Golbat Vitamins and Berries. Every time you feel it a Berry or a Vitamin to raise one of its stats, its Happiness will also rise.
Keep boosting Golbat's Happiness until it rises above 200 to "MAX." As soon as its Happiness is high enough, it will automatically evolve into Crobat.