Since Void War currently has no built-in mod support, any mods you can install will take the form of an xdelta file, which will directly modify the data.win file in your installation folder. Before applying an xdelta patch to your data, you should make a backup. To uninstall a mod, you can replace the game data with the backup you made. Mods are not cross-compatible, only one xdelta patch may be applied to the game at a time.

To apply an xdelta patch, first get the latest release of DeltaPatcher[github.com]. Download the Windows zip file, extract the application and run it. For the "Original file" field, navigate to the folder where Void War is installed and select data.win. If you don't know where Void War is installed, you can find it by right-clicking the game in your Steam library and selecting "Manage" > "Browse local files". For the "XDelta patch" field, select the xdelta file you want to patch. Once you’ve selected the files for both fields, press the "Apply patch" button to install the mod.

Void War is a GameMaker Studio 2 game compiled with VM. That means the best modding tool that exists for it right now is UndertaleModTool[github.com]. If you have experience with GameMaker, UTMT will be relatively intuitive to use. Some features are missing, such as the ability to set the nineslice for a sprite or add tags to assets, but these can be done using GM's built-in functions instead. I maintain detailed notes on how I create my mods in this document: https://docs.google.com/document/d/1qNb5fkbYisOytJpLOxbfWRKzDVEMcytn6fWzX04fVs0