From what I remember you need a base theme, and an image editor to edit the images, then you have to use Theme Brlyt Editor to edit the Brlyt of the theme to make an mym file, and then ThemeMii to download the base app file for your Wii region and version, to create a csm for your theme, then you need MyMenuify to add the theme to your Wii.
There are YouTube tutorials for this, that explain this in more detail, but this is the best explanation I can give for now.