Common Issues
My item is just a black and purple square
You haven't created an item JSON (the one in assets/<modid>/items/ folder). All items must have an item JSON.
See here for instructions on creating and placing the item JSON file.
My item is completely invisible
You forgot to apply the GeoItemRenderer to your class. GeckoLib can't render your item if you haven't told it how!
See here for instructions on applying the item renderer, or here for instructions on creating one.