The easiest way to create a mask is to run /em create <name> in-game. After you have ran the command just open your masks.yml and edit the values within the file for that mask.
Example Mask
dragon:name:"&4&lDragon Mask"lore: - '&c+5% DMG' - '&cImmune to fire and lava damage' - '&7The decapitated skull of a slain' - '&7Timeless Dragon from the Ender Dimension' - '&6* Timeless Dragon Update' - '' - '&7&oAttach this mask to any helmet' - '&7&oto give it a Visual Override!' - '' - '&7To equip, place this mask on a helmet.' - '&7To remove, right-click helmet while attached.'skin:"74ecc040785e54663e855ef0486da72154d69bb4b7424b7381ccf95b095a"applied_lore:"&4&lDragon Mask &c(Immune to lava and fire damage)"effects: - 'Damage:5' - 'Immunity'