First, install the latest BepInEx 5 in the game folder.
Next:
- put CastleStory_CustomKeybindingsPatcher.dll file in {Game}/BepInEx/patchers folder
- put CastleStory_CustomKeybindingsPlugin.dll file in {Game}/BepInEx/plugins folder
- put keybindings.lua file in {Game}/Info/Lua/modconf (create that folder).
See the contents of lua to learn how to use it.