How do you win a dress up competition?

Competitions are also easy, I am assuming you are referring to the stylist arena? That is pretty easy to do, also you need to do is look at the theme and pick an outfit that matches that theme.

Also, how to win and get S's on the competition, look at Momo's reminder on your left when you are changing your outfit for the competition. Then go to S tips and wear the clothes that are listed. I have tried it before and it works.