Mob Skull Shaders allow resource packs to apply a Post Processing Shader when an item is worn on the player's head. To start, create a new JSON file in the assets/{namespace}/subtle_effects/mob_skull_shaders folder. Next, inside the file create an object called item along with an element called id. id is where you specify the item ID used for this effect

{
  "item": {
    "id": "minecraft:diamond_helmet"
  }, 
  ...
}

Additionally, an optional list of item components required for the shader to be applied can also be specified.

Note: Specifying components is not available in 1.20.1 and due to limitations with that version NBT tags are also not available

{
  "item": {
    "id": "minecraft:diamond_helmet",
    "components": {
      "custom_name": "{\"text\": \"Cool Helmet\"}"
    }
  },
  ...
}

Lastly, add an element called shader. This is the id of the post shader to be applied.

{
  "item": {
    "id": "minecraft:diamond_helmet"
  },
  "shader": "minecraft:spider"
}