To install just paste the jollyrogerf18.pak file into a ~mods folder in your ace combat 7 installation folder

for example: 
D:\Program Files\Steam\steamapps\common\ACE COMBAT 7\Game\Content\Paks\~mods

If you have no "~mods" folder then create it in place. don't forget the "~"

This skin can only be used if you completed the campaign at least once.

More mods at: https://www.nexusmods.com/users/37192780