Mine ancient debris in the Nether (at Y-levels around 8–22)
Smelt ancient debris in a furnace to get netherite scraps
Combine 4 netherite scraps with 4 gold ingots in a crafting table to make 1 netherite ingot
Obtain the armor piece you want to upgrade (diamond helmet, chestplate, leggings, or boots)
Find or craft a smithing table
Place the smithing table UI items:
Top slot: diamond armor piece
Bottom slot: netherite ingot
Result: netherite armor piece
Take the upgraded netherite armor from the output slot
Repeat for each armor piece as needed