Instructions
Fly to Goldenrod City and enter the Goldenrod Underground portion of the city.
Walk to the barber shop in the Goldenrod Underground and talk to whichever brother is working. The younger brother works on Sunday, Wednesday and Friday and the older brother works on Tuesday, Thursday and Saturday. The shop is closed on Monday.
Pay the younger brother $300 for a haircut or the older brother $500 for a haircut. Choose which Pokemon you want to give a haircut and confirm your selection. Your Pokemon's happiness level will increase after the haircut is finished.