Find a Bastion Remnant in the Nether
Search the Bastion Remnant for chests
Open chests until you find a Netherite Upgrade Smithing Template
Looting more Bastion Remnants increases your chances of finding one
Use the template in a Smithing Table to upgrade diamond gear to netherite gear
Duplicate the template by combining it with diamonds and netherrack in a crafting table, if needed