assets/mod/blockstates/BlockSquare.json

{
    "forge_marker": 1,
    "defaults":
    {
        "textures":
        {
            "all": "firstmod:blocks/square"
        }
    },
    "variants":
    {
        "normal":
        {
            "model": "cube_all"
        },
        "inventory":
        {
            "model": "cube_all"
        }
    }
}