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.
FAQ
Should I use a Joker Card immediately?
Usually no. Wait until normal drops have slowed down and you know which missing card blocks the best visible reward set.
Which card is best for a Joker Card?
Prefer the rarest missing card in the reward set that actually improves your progress. Do not spend it on a card that is likely to appear naturally.