To start off your quest to become a Ninja in Club Penguin, go to the Dojo Courtyard, located on your Map.
Once inside the Dojo, walk to the green mat under the red tower and talk to the Sensei.
How it works
Sensei will explain how the game works. It’s similar to Rock-Paper-Scissors. He will explain the specifics. Remember, a yellow card makes it so that the smaller number beats the bigger number the next turn (if they’re the same element).
How to win
A penguin must win three turns with three different elements. The cards used to win these turns must all be different colors as well.
Belts
Once you’ve played a certain amount of games, you will get a new belt. Belts denote how strong of a player you are and are wearable.
Earning Experience
Talk to Sensei again and select Competition Mode. Keep playing these kind of games because they give you more experience than Practice Mode so you will gain new belts faster.
Guidelines
Here’s an estimated guideline for how many wins you need to advance belts. Loses do in fact help you advance as well, but not as much. These are all estimations.
White belt : 1-3 wins
Yellow belt : 1-6 more wins
Orange belt : 2-10 more wins
Green belt : 3-10more wins
Blue belt : 4-10 more wins
Red belt : 5-10 more wins
Purple belt : 6-10 more wins
Brown belt : 8-10 more wins
Black belt : 10-17 more wins.
Helpful Tip
Try to anticipate what the opponent is going to do. If he/she only needs a Water to win the game, then you better keep using Ice!
Cheats
Here’s a little cheap, useful trick to use when playing Card Jitsu matches on Club Penguin.
How to beat Sensei
So you’ve gotten the Black Belt. But where’s the Ninja Headband? You gotta beat Sensei first! Talk to him and then select the option to play him in a game of Card Jitsu! For the first 3-5 matches, he will probably win EVERY SINGLE turn. Just let him think he is the Master of Club Penguin and continue playing him. Around the 4-6th matches, he will slip and lose a turn. From then on, he will continue his losing streak. You’ll either win the match or win the match after that. Now who’s the Master of Club Penguin?

Once you beat the Master
Once you beat Sensei, he will give you the Ninja Headband. Now you can enter the secret Club Penguin Ninja’s Only Room! Walk outside of the Dojo and go through the secret Rock Door.
Inside the Mysterious Room
The room inside looks pretty cool! There is a catalog in there that has a Ninja Igloo, Ninja furniture, and most importantly: NINJA clothing! Once you have enough coins, buy the Ninja outfit.

Becoming Invisible
Ah, yes. Back a LONG LONG time ago, [See my History of Club Penguin for more details], Ninja Penguins could become invisible. Well, now they can again! Put on your Ninja Costume and Headband and nothing else. If you dance, you become transparent!
Now what?
Show off your accomplishments! Parade around Club Penguin with your new Ninja costume, belt, or headband. The Ninja costume (or belt or headband) greatly complements other Club Penguin clothing items. Also, having the item shows you are a dedicated, hardcore Club Penguin player. I mean you have to be pro to spend so much time on Club Penguin to get the outfit. Other penguins will worship you. Too bad there aren’t any real cheats that would have sped things up.