To create and add a custom splash screen to your Wii U, you will first need to have aroma cfw and sdcafiine installed to your Wii U aroma.foryour.cafe after setting up both you can head over to the theme cafe discord (Discord | #【🎨】splashes | Theme Café) and grab your splash screen. in your sd card go to wiiu/environments/aroma and put the png file there, make sure the file is renamed to splash and is a .png file. put your sd card back in your Wii U and boot your Wii U with the aroma environment, if you see the splash you chose appear then Hooray! You Did It! But if not then make sure you double check this guide and you can also ask over on the theme cafe discord since they're always willing to help!