There are two main ways to mod Paper Mario: Star Rod Classic and Diorama64. They work so differently that most of the documentation from here on applies to only one of them, so choose one before you start.
Continue with Star Rod Classic A well-established set of tools for editing Paper Mario. Deeper changes to the game can need assembly code, and it no longer gets major updates.
Continue with Diorama64 An editor for Paper Mario DX, a base for making mods that is built on the Paper Mario decompilation. The game is C code instead of assembly, so deeper changes are often easier. It's alpha software, so expect bugs, and fewer people can help.
Which to choose
Both can make large mods. Star Rod Classic is mature, and more people know it well enough to help you. Mods made with it go well beyond the base game, but the further a mod moves from the game's own mechanics, the more assembly it takes.
Diorama64 makes changes that would need complex assembly in Star Rod Classic much easier, such as new partners, status effects, or abilities in the world. Updating your mod to a newer version of Paper Mario DX can mean resolving conflicts by hand.
- If you want a mature tool that many modders know, Star Rod Classic is a good fit.
- If you want to change how the game works, or you already know C, Diorama64 is a good fit.