We recommend using the Thunderstore Mod Manager or an alternative for installing mods
An addon to the Custom Items SDK for better functionality with ModThatIsNotMod.
Extract the zip into the assets folder of your Custom Items SDK. If you don't have that, download it here: https://boneworks.thunderstore.io/package/gnonme/CustomItemsSDK/
Instead of using the ItemSettings editor window, add the CustomItems component onto your CustomItems gameobject.
There's a readme file included with more info about using custom monobehaviours.