You don't. When you have used all three costume positions, you simply "save" over any of the existing ones, replacing and erasing the one that was already there.
you use it to save costumes and if you want to you have to buy it it is 250 credits you save up to 3 costumes and you click customize if you want to wear it.
The costume collector, when it was available, would store three of your "costumes" or appearances that you could switch between. Now there are ready-made costumes at the Store that are...
There are numerous costumes available for purchase in the Poptropica Store, as well as by copying individual Poptropica characters on most islands. The ad-games that appear temporarily will also...