• Copy the link in config.yml under the property default_url
  • open it in your browser and download the file
  • Extract the file in %appdata%\.minecraft\resourcepacks\ItemsAdderResPack (or name it how you want)
  • open Minecraft, in the resourcepacks options select ItemsAdderResPack
  • open config.yml and set apply_on_join to false while you are editing the resourcepack locally
  • join the server, then start editing the files inside ItemsAdderResPack
  • After you've edited something in the resourcepack, to reload the resourcepack press F3+T on your keyboard

Publish the resourcepack

  • set apply_on_join to true
  • set use_default to false
  • zip the ItemsAdderResPack folder and upload it on an hosting website
  • set custom_url to your uploaded file download link
  • reload plugin
You must upload the resourcepack .zip file on a hosting service that allows you to direct download the file (which doesn't shows a "click here to download" button or a download page).
You can upload it on Dropbox, get the share link and change ?dl=0 at the end of the link to ?dl=1