Hello! Welcome to the guide on how to install these textures for Patapon 2 using PPSSPP. I'll try to keep this as simple as possible for people who are not familiar with this.
PPSSPP has an option to replace shown textures with any given image. We call this texture replacement. To enable texture replacement in PPSSPP, open Patapon 2 in PPSSPP, then open the emulator pause menu. Go to settings, then the Tools tab, then click on Developer Tools. Scroll all the way down and enable "Replace textures". Click on "Create/Open textures.ini file for current game" to create the TEXTURES folder for PPSSPP. You can find the TEXTURES folder in the folder directory memstick/PSP/. There will be another folder inside the TEXTURES folder. The name of the folder changes between game versions, but it should start with "UC". Check the README.md for game versions. You can drop textures.ini and the textures folder in there to install the textures. Make sure to reload the game so the changes can be made!