Board Thread:Game Discussions/@comment-5158113-20131120135947/@comment-24108837-20131120184144

You have to first upgrade your first armour to level 35. You will get a notice that you can now craft the + version.

Then you go to your Armorsmith and make sure the name is now "Behemoth's Vanguard +" You have to see the plus! Then you can start crafting the new armour. You don't have to do that before the event ends. The plus version is a standard property of craftable armours and has nothing to do with Behemoth as such. You can get plus versions of other craftable armours too in the same way. Even Basic Earth Armor has a plus version.

Warning: do not start crafting the 2nd armour before you see the + in the name. You will not get the + later if it isn't there when you start crafting.