Ok... here's how to do:
From the Wall Market entrance, go to the right to see the Honeybee Inn. Talk to the fat guard and ask where Tifa is. He tells you that she is at Don Corneo's mansion. Go back to Wall Market, and head up two screens through the mansion entrance. Talk to the guard here, and he says that the Don is only interested in girls, so Aeris tells you secretly to dress up as one. Go back down two screens, then enter the shop with the clothes racks outside.Talk to the girl in the counter. You find out about her father and his drinking problem. Go back up one screen, and enter a door to the left, which leads to a bar. Talk to the man sitting close to the exit alone. He is the drunk father of the girl at the clothes shop. When Aeris tells him of your plan, he immediately accepts the job and help help you to design Clouds dress and stuff.

After Cloud tries it on in the fitting room, there's something that still is missing (or something like that) go up one screen, and talk to the small guy near the boxing-house-thing. You must choose to challenge someone in a squatting match. They will explain how to do. No matter if you win or lose... you will still get the thing that's missing.

You should do something like that if I remember coorectly.