Use it on the rarest missing card
A Joker Card is most valuable when it completes a high-reward set that has one hard card missing.
Wait until the event is late
Using it too early can waste value if the card drops naturally later.
Do not spend it on a low set
A small reward set usually does not justify using the strongest card tool.
Screenshot before using it
Save the set screen and reward screen so the result can be checked later.