How to make a Tier
Default configs
The default configs can be found here.
How to add tiers
Each tier is its own config file, placed in the /tiers/ folder, and you can add or remove them as you please. There's an example config called _example.yml to help you out!
The ID of the Tier is the file name. This is what you use in commands and the defaultTier section of the set configs.
ID's must be lowercase letters, numbers, and underscores only.
Example Armor Set Config
display: "&c&lNETHERITE" # The display in-game
requiresTiers: # If this tier requires a prior tier
  - diamond # Tier ID
  - iron
crystal:
  item: end_crystal # The crystal item, read more here: https://plugins.auxilor.io/all-plugins/the-item-lookup-system
  name: "&cNetherite Upgrade Crystal" # The name shown in-game.
  lore: # The lore shown in-game. Set to `lore: []` to remove lore.
    - "&8Drop this onto an armor piece"
    - "&8to set its tier to:"
    - "&c&lNETHERITE"
    - ''
    - "&8&oRequires the armor to already have Diamond tier"
  craftable: true # If the armor piece is craftable
  recipe: # The recipe, read here for more: https://plugins.auxilor.io/all-plugins/the-item-lookup-system/recipes
    - air
    - netherite_ingot
    - air
    - netherite_ingot
    - ecoarmor:upgrade_crystal_diamond
    - netherite_ingot
    - air
    - netherite_ingot
    - air
  giveAmount: 1 # Optional, set the amount of items to give in the recipe
properties:
  helmet:
    armor: 3 # The armor attribute
    toughness: 3 # the toughness attribute
    knockbackResistance: 1 # The knockback resistance attribute
    speedPercentage: 0 # The movement speed attribute
    attackSpeedPercentage: 0 # The attack speed attribute
    attackDamagePercentage: 0 # The damage attribute
    attackKnockbackPercentage: 0 # The knockback attribute
  chestplate:
    armor: 8
    toughness: 3
    knockbackResistance: 1
    speedPercentage: 0
    attackSpeedPercentage: 0
    attackDamagePercentage: 0
    attackKnockbackPercentage: 0
  elytra:
    armor: 3
    toughness: 0
    knockbackResistance: 1
    speedPercentage: 0
    attackSpeedPercentage: 0
    attackDamagePercentage: 0
    attackKnockbackPercentage: 0
  leggings:
    armor: 6
    toughness: 3
    knockbackResistance: 1
    speedPercentage: 0
    attackSpeedPercentage: 0
    attackDamagePercentage: 0
    attackKnockbackPercentage: 0
  boots:
    armor: 3
    toughness: 3
    knockbackResistance: 1
    speedPercentage: 0
    attackSpeedPercentage: 0
    attackDamagePercentage: 0
    attackKnockbackPercentage: 0
Understanding all the sections
Tier Config
display How the tier will show up on armor pieces
requiresTiers A list of tiers that the item must be in order to apply the crystal.
To explain this, here is an example progression:
display: "&c&lNETHERITE" # The display in-game
requiresTiers: # If this tier requires a prior tier
  - diamond # Tier ID
  - iron 
The "Netherite" Tier can only be applied to armor pieces that have the "Diamond" or "Iron" tier.
Crystal
item: The base item, read here for more: Item Lookup System.
name: The item name in-game.
lore: the item lore shown in-game. Set to lore: [] to remove all lore lines.
craftable: If the item should be craftable (true/false).
recipe: The recipe, read here for more info: Crafting Recipes
giveAmount: The amount of items to give when crafted.
Properties
properties: are the actual attributes given to an armor piece with that tier. You can find default attributes on the Minecraft Wiki