Welcome to the New Super Mario Bros. U Modding Wiki! To begin creating a mod, first you will need the New Super Mario Bros. U disc, and a homebrew-enabled Wii U. A guide for the latter can be found here. After you have completed that step, you may continue to the next guide, Dumping Wii U Games.
If you are looking for how to mod New Super Mario Bros. Wii, it has a separate modding wiki found here.