To access this feature, open up the Catwalks tab like normal and then click on the Costume Contest subtab. There you'll see the option to either vote on today's submissions or enter the contest for the next day. You can see what theme the contest requires you to dress up as, and then deck your avatar out in your best clothes in an attempt to be the best looking avatar who most accurately represents the theme in style.
If you'd rather vote on your favorites, you can go through matches of threes and pick the best outfits. Once you vote, you can see who is winning so you can see how you matchup with other people's opinions. You can also ask your friends for votes, which adds to the social aspect of the game.
Finally, if you have a brilliant idea for a costume theme you can submit it in the available box and they will choose your theme if they like it. Pretty neat!
Are you enjoying the new Catwalks addition in Sorority Life? What themes do you hope they add next?