You would do this with model files and parents. For example, for carved pumpkins you could do this:

---

{
  "parent": "minecraft:block/cube_bottom_top",
    "textures": {
      "top": "minecraft:block/pumpkin_top",
      "front": "minecraft:block/carved_pumpkin",
      "side": "minecraft:block/pumpkin_side",
      "bottom": "minecraft:block/pumpkin_bottom"
  }
}

---

You can find out how to do more like this here:
The Block Model JSON