When I started modding PSP games, mainly to swap out textures and music, I found a few tools that really made a difference:
UMDGen: This tool is a game-changer for working with PSP game ISO files. It lets you pull out files from an ISO, or put new ones in, which is exactly what you need for changing textures or tunes.
PPSSPP: This one is mostly known as a PSP emulator, but it's a lifesaver for testing mods. You don't have to keep moving files to your PSP to see if your mod works. Plus, it has this cool feature where it can load textures from a folder on your computer, making texture mods a breeze.
CRI Packed File Maker: Some PSP games pack their files in a special way using CRI Middleware. This tool helps you open those packs, so you can change stuff inside, like textures or sound files.
Each of these tools played a crucial role in my modding adventures. They might seem a bit technical at first, but once you get the hang of it, they're pretty straightforward and incredibly powerful.