create a mod-loader folder to VampireSurvivors\resources\app.webpack\renderer download all these file and move to mod-loader folder that you created
there's an example of mod in mods folder (goku and its weapon ki)
copy and replace main.bundle.js or modify main.bundle.js your own
add these code to the top part of the main.bundle.js
const { Mod } = require("./mod-loader/mod.js");
const mod = new Mod();
then find _0xc95609 = _0x3862c8["O"](_0xc95609);
then add this mod.inject(_0x3862c8);
it should look like
_0xc95609 = _0x3862c8["O"](_0xc95609);
mod.inject(_0x3862c8);
})();