How do I use them?
I have a few of these in my Inventory from previous events. I have yet to figure out how to use these though. I've asked a few people in-game and they replied one of two things:
1) It needs to be a cash or relic item
2) You need some manual (from the cash shop?)
I looked in the shop for a "manual" and I haven't found any. As for #1, I don't have a cash or relic item to check the solution with.
It's only cash items that you can use it on, and some event items I believe. Just select the Prop button next to the Sell button.