minecraft-assets/data/minecraft/recipes/diamond_leggings.json

{
  "type": "crafting_shaped",
  "pattern": [
    "XXX",
    "X X",
    "X X"
  ],
  "key": {
    "X": {
      "item": "minecraft:diamond"
    }
  },
  "result": {
    "item": "minecraft:diamond_leggings"
  }
}