model file (models/block/stable_pulsating_brain_stone.json):

{
    "parent": "block/cube_all",
    "textures": {
        "all": "brainstonemod:blocks/stable_pulsating_brain_stone"
    },
    "ctm": {
        "model": {
            "model": "cube"
        },
        "face": "brainstonemod:blocks/stable_pulsating_brain_stone.cf"
    }
}

ctx file (textures/blocks/ctm/stable_pulsating_brain_stone.ctx):

{
    "type": "CTM",
    "textures": [
        "blocks/stable_pulsating_brain_stone",
        "./stable_pulsating_brain_stone_ctm"
    ]
}