Quick guide for people familiar with resource packs and Jsons.

Important Folders:

JSONs and Banner images go into the .../entity/banner folder. Shield images go into the .../entity/shield folder.

Clients and Servers need the JSONs. Images are only required on the Client side, however it will not crash a server if they are included in the config folder.

Json Format:

{
  "item": "minecraft:Item_Needed_To_Create_Design",
  "name": "Name_of_PNG_file",
  "id": "ID",
  "localized": "Engilsh_Name_of_Design"
}

The item field is the name of the item, identical to using the /give command

The IDs should all be unique, and are registered with "bl_" before them. IDs should be only 1-4 letters.

Localized is the English name given to the design. If the banner was of a ball, localized should be "Ball". Localized is used in the tooltip to describe the design

Name is the PNG file without the suffix .png . If this is not properly set, the textures will fail to load.