1. Download
JGecko U.jar.
2. Make sure you have
JRE (Java Runtime Environment) installed.
3. Double click JGecko U.jar like any other *.exe file (JGecko U will create files and folders in its current directory on the first start so you may want to place it in a separate folder).
4. Put in the IP Address of your Wii U into the "IPv4 Address" field and press the "Connect" button.
5. Press the "Add Codes" button in the "Codes" tab.
6. Enter the name of the cheat code (in the "Title" field) and the code itself (in the "Code" field). Press "OK".
7. Repeat step 5 and 6 if you have more codes.
8. Check the codes you want to activate and press the "Send Codes" button.
The codes should now be active in-game.
(Thanks tastymeatball for this post)