There are many ways to modify Sonic Adventure DX and Sonic Adventure 2: Battle on PC using the Mod Loaders, from basic file editing to powerful code injection.

The first step is to create your mod using the Mod Manager. This will create a folder in which you will be able to put your modifications without affecting the original game files or executable.

From there, you will be able to do:

Creating a mod

To create a mod you need the SA Mod Manager with Sonic Adventure DX and/or Sonic Adventure 2: Battle set up.

First open SA Mod Manager with the game of your choice and click on the "Add Mod" button in the Mods tab.

Then select "New Mod (For Developers)"

This opens the mod creation form.

Mod Information

Here are the information requested (* is mandatory):

Name*: the name of the mod
Author: your nickname, it is acceptable to add co-authors separated with commas
Description: short description of the mod, it appears below the mod list
Version: a version number, preferably numbers and dots. A common way to represent version is "major.minor".
DLL: for code injection, this is the name of the DLL with the extension (e.g. "my-mod.dll")
Category: one of the categories listed or none
Mod ID*: a unique name for your mod that should not change, preferably without spaces (e.g. "game.author.modname")
Include Directories: this is used for codeless mod configuration
Author URL: link to a page of your choice, ideally showcasing your mods (GameBanana or GitHub profile for example)
Source Code URL: for code injection, link to the source code (GitHub repository for example)

This should have created a folder with a system folder (for SADX) or a gd_PC folder (for SA2B), you can use these for file replacement.

Update information

This tab is for automatic updates, it is generally filled out later.

For GitHub:

Author and repo is the right part of the repository URL (e.g. myself/my-mod)
Release Asset is the name of the zip file in the release (e.g. my-mod.7z)

For GameBanana:

Mod ID is the number in the mod's page URL (e.g. 123456)
Mod Type shouldn't be changed (legacy)

For self hosted:

Update URL: path to mod folder root on a remote http server
Changelog URL: path to changelog file (plain text)

Cheat codes

This tab is for cheat codes.

Dependencies

This tab is for mod dependencies. If your mod absolutely relies on another mod to function properly, you should add it here. The mod manager will warn users if they lack a dependency.