Check for the ID/s of the item/s and/or monster/s inside the mod’s directory, looking inside the.json files, and then you add the ID/s inside the tile_config.json for the tileset.
Also for actually creating the tiles, I myself use Paint.net and a layered image, with a checkered guide on the background so I know the limits for each tile slot. After I am done making changes I remove the guide on the background and save only the images for the tiles, that’s what I meant to say.