Find a Bastion Remnant in the Nether
Loot chests inside the Bastion Remnant
Obtain a Netherite Upgrade Smithing Template
Use the Netherite Upgrade Smithing Template in a Smithing Table
Combine it with a Diamond item and a Netherite Ingot
Upgrade your Diamond gear to Netherite gear
Duplicate the Netherite Upgrade Smithing Template with 7 Diamonds and 1 Netherrack block in a crafting grid
Use the duplicated template to upgrade more items