How do I use the Joker?

You get one joker for every level so use it wisely.
The joker can be used to match any card so is very useful.
Most times it is better to try to save the joker card as long as possible and only use it if you need to.
Use the Joker to match difficult cards.
If you have been getting a lot of the same number cards turning over in the deck and later you find you need that card to match cards on the board, that's a good time 
to use the joker.
See example.
Here you have 2 jacks under the queen and you have been turning over a lot of kings, so you may not get a card to match the queen before you run out of cards. 
This is a good time to use your joker to clear the queen and give you a better chance of finishing the game.
Use the joker to continue a streak.
If you have a long run going and are missing one card to continue the joker can be used so you don't have to abandon the run. 
Here I already have a run of 8 cards and need 12 to complete the level so I can use my joker now and continue removing at least 4 cards to complete the task.