Once the plugin is up and running, to add custom skins to skulls here is what you need to do!

find the item: section in the slot that you want a custom skull skin.

Example of base64, to have a yellow cupcake:

To do a Base64 skin, you can do: base64-value.

item: "base64-eyJ0ZXh0dXJlcyI6eyJTS0lOIjp7InVybCI6Imh0dHA6Ly90ZXh0dXJlcy5taW5lY3JhZnQubmV0L3RleHR1cmUvNGMzZjRhYjdmYTg0NzY0ODAxYjZmODg3NGFhMzk2MjhhMTNkOGJmN2MzNWRlMjE4NzJlMmQwYzc5MzlkM2NmZCJ9fX0="

Examples of non-base64:

To do a player's skull, you can do: head-playername, or if you want it to be the player who opened the GUI's skin, you can do head-{playername}.

item: "head-The_PizzaLover"

item: "head-{playername}"