Everything here is already set up for it, just answer the two questions and roll. If you want to build your own variation, there is a fine-tune button under the card.
Dress or top?
Pick one to build around.
Which pieces count?
This only changes how many pieces the randomizer draws from. There are 83 free-to-play dresses, for example, so Free to play rolls a number from 1 to 83. Including banner pulls makes it 1 to 185.
You will get numbers, not named items. Count to each number in your own closet, so the same roll lands on different pieces for everyone.