How to Craft Netherite-Diamond Armor
Make the ingots
Netherite-Diamond armor starts with Netherite-Diamond Ingots. Combine a Netherite Ingot with a Diamond in a smithing table to produce each ingot. A full set of four armor pieces needs four ingots, so craft at least that many before you begin.
Craft each piece
Craft Netherite-Diamond Helmet, Netherite-Diamond Chestplate, Netherite-Diamond Leggings, and Netherite-Diamond Boots. Each uses Netherite-Diamond Ingots arranged in the standard armor shape for that slot. Craft them in one sitting so you do not lose track of how many ingots you have left.
Pick a different tier if you want
Armor exists in all four tiers. Swap the ingot to make Netherite-Emerald, Netherite-Gold, or Netherite-Iron armor pieces. The shapes are identical; only the ingot changes. This is useful if you want to save Netherite-Diamond Ingots for tools and run a cheaper tier on armor, or the reverse.
Equip and store
Equip the four pieces in their normal slots. Keep a spare Chestplate and Boots in a chest near your bed so a death does not leave you unprotected. Because each piece costs an ingot, restock your Netherite Ingots and Diamonds before crafting replacements.