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

{
  "type": "minecraft:smithing",
  "base": {
    "item": "minecraft:diamond_leggings"
  },
  "addition": {
    "item": "minecraft:netherite_ingot"
  },
  "result": {
    "item": "minecraft:netherite_leggings"
  }
}