GUI

Everything is easier with interactive inventory.

Manual, how to configure items at GUI.

Locale/GUI.yml

Configuration file, where all data about items are located.

Restriction: #Item's key. Maintained by system, can't be changed.
   HeadName: 
     EN: "&2&lRestrictions"
   Lore:
     EN:
       - "&6\u25CF &7List of players, who can apply"
       - "&7voucher item."
       - "&6\u25CF &7Keep empty, to allow all players"
       - "&7to apply voucher item."
       - ""
       - "&6\u25CF &3Players &6\u00BB &7{voucher_item_restriction_player_list_size}"
       - "&6\u25CF &3Permission &6\u00BB &7{voucher_item_restriction_permission_activity}"
       - ""
       - "&6\u25CF &7Click"
       - " &2\u2022 &7Manage requirements"
   Material: "PAPER" #Name of the material
   MaterialData: 0 #Metadata of the item. Especially color. Only for 1.8.8 - 1.12.X
   Slot: 3 #Slot number where item will be played in inventory
   SkullData: "textures" #Texture value of player's head. 
   ItemFlags: #Flags applied to item.
   - HIDE_ENCHANTS
   CustomModelData: 0 #Is an integer that may be associated client side with a custom item model. Supported at 1.14.X and aboove.

ItemFlags

• HIDE_ENCHANTS

• HIDE_ATTRIBUTES

• HIDE_UNBREAKABLE

• HIDE_DESTROYS

• HIDE_PLACED_ON

• HIDE_POTION_EFFECTS

• HIDE_COLOR - 1.16.X and above

Helpful stuff

Materials: https://hub.spigotmc.org/javadocs/bukkit/org/bukkit/Material.html

Textures of player's head: https://minecraft-heads.com

Grid can help you to compute right slot index.