You can put items on your model by clicking the box they are in. They will appear on your model and on a list below the model. Clicking the item box again removes it from the model and from the list.
There's a slider next to your model. Move it up and down and the item that is highlighted on the list below your model gets moved in front and behind of the other items. You can add as many items on your model from as many categories as you want.
When your outfit is ready, press Save.
If you're sure you don't need an item anymore, you can delete it by clicking the garbage bin under the item. Before deleting an item, make sure you don't need it anymore, as you can't get it back.
If you are a VIP, you can auction your items. Read more about auctioning items from auctions help page.