The Advent Tree Instructions are required to unlock access to the Advent Tree. They were in the December 2009 and November 2017 Account Upgrades. For years and years, they have been circulated in and out of the Account Upgrades Shop.
How to Get
From Account Upgrades Shop or Monthly Mission Contest
The instrution pieces can also be purchased from user shops, trades and auctions.
Is there an avatar?
Yes, there is an avatar for completing the instructions. Go visit the Advent Tree page afterwards to claim it.