How to tame a nautilus and get nautilus armor in Minecraft
For netherite, put a Netherite Upgrade Smithing Template, the Diamond Nautilus Armor and a Netherite Ingot in a Smithing Table (see how to get netherite ).

The Netherite Upgrade is the smithing template that turns diamond gear into netherite gear at a Smithing Table.
It is made with the Crafting Table from 7 × Diamond, Netherrack and Netherite Upgrade.
Place the template, a diamond item and a Netherite Ingot in a smithing table: the item becomes its netherite version and keeps its enchantments and name. It works on diamond armor, tools, swords and spears, and on Diamond Horse Armor and Diamond Nautilus Armor.
The template is used up each time. The treasure chests of a Bastion Remnant always hold one, other bastion chests sometimes do, and seven diamonds with a block of Netherrack copy it into two.
7 × Diamond + Netherite Upgrade + Netherrack → 2 × Netherite Upgrade
// Slots with several items cycle through every accepted option.
For netherite, put a Netherite Upgrade Smithing Template, the Diamond Nautilus Armor and a Netherite Ingot in a Smithing Table (see how to get netherite ).
For netherite, put the Diamond Horse Armor, a Netherite Upgrade Smithing Template and a Netherite Ingot in a Smithing Table (see how to get netherite ).
The Smithing Template "Netherite Upgrade" is not craftable from scratch:
At the Crafting Table: 7 × Diamond, 1 × Netherrack and 1 × Netherite Upgrade give 2 × Netherite Upgrade. It can be found as loot in Bastion Remnant.
It upgrades diamond gear to netherite at a smithing table. It is an ingredient in 12 recipes, made at the Smithing Table; for example Netherite Helmet, Netherite Hoe, Netherite Horse Armor, Netherite Leggings, Netherite Nautilus Armor and 4 more.