How do I use the costumes? I don't understand. Can someone tell me?
First you need to ask them during a date what kind of play they're into. 
Once you figured out which one of them has costume sex position available, you can go to mall to buy the necessary stuff. You need to match the outfit and the place where you had sex on.
In this case, Caroline liked cosplay. So place a couch and outfit brawler pole besides it. As you can see above, there's heart on it means you can start the h-scene. Hope this helps