What you need to do to get the achievement is not very hard, but "officially and also to make it feel right", it should only be done during the offical Croteam X-Max event - starts with December and ends in January. It is possible to get the achievement during another time of the year. But this guide is not about that.


Turn on the game (difficulty does not matter, the tree is there independent of the difficulty you play - if you just want to make a run for the achievement, play on tourist), and play through the first level until you reach the museum. On your way there you will already notice christmas presents on the ground.

IMPORTANT before you start killing: The achievement requires you to decorate the tree with certain "body parts you get from melee attacks of certain enemies".

Gnaar Eye: 3 Gnaars will come at you when you enter the area in front of the museum. I recommend to melee attack one immediately. Walk (DON'T run, running will make you drop the eye) to the tree and throw it on it.
Soldiers heart: To rip out a soldiers heart, you have to hold the weapon "sledgehammer". If you have the gun and melee attack you will just kick the soldier away. So rip out one heart and also throw it on the tree.
Kleer head: At the end of the level one single kleer will spawn, then 3 others. There is a little time between that. So let the first kleer spawn and SEE you. It will follow you to the tree. Rip off its head and throw it on the tree.

Done. As soon as the third part is thrown on the tree the achievement will pop up and the legendary "jingle bells remix" from land of the damned will start to play. I don't think that the order in which you throw the parts on the tree matters, but since I can only get the achievement once I cannot know for sure. I did it in the order as the emenies are listed above.

The tree in front of the museum and already decorated.

As you can see, still works. Got it today.

And a very serious Christmas to you all.

This achievement also still works. The difference to "Christmas in Cairo" is, that you have to decorate the christmas tree in every level - yes, there is one in every level. The "decoration" is the same as "Christmas in Cairo":
Gnaar eye, Kleer head and Soldiers heart. First thing that will probably come to your mind - not all those enemies are available in all levels. True, but no worries. You will notice presents on the ground. There will always be 3 presents in the "area around the tree", picking them up will spawn the necessary enemies to decorate the tree. The rest of the presents that are scattered around the levels are "hp" and "armor". Also you will notice banners and other decoration everywhere.

So there are 2 options: When the enemies are available use them, if not look for the presents and pick them up. If you take the "present way", I highly recommend to clear the area of all enemies first. In the action it will happen that you accidently killed the ones you need. No running after the melee attack when you have the "eye", "head" or "heart". You will drop it and you cannot pick it up again.
I will not tell you the exact location of the trees, but they cannot be missed, just play through the game.
Every time the last "decoration" has been thrown on the tree, the "jingle bells" remix starts to play. You will also hear enemies spawn and scream and even some shooting. But no additional enemies spawn - little sound trap.
I don't know if "save files" created on 24th or 26thwould work to get the achievment if you would e.g. load them on 07.01.2021.
I also don't know if loading the single levels and just playing till the tree location, saving, loading another level, etc. would work to get the achiement.
Update, 20.12.2022 about "save files":
According to User "puddingtopf", save files work. Thank you very much for sharing that information in the comments!
If anybody has valid infos about "save files" and "loading single levels", you are welcome to leave a comment and I will update the guide.
Depending on your personal skill, this achievment can get tricky on difficulty higher than "normal".

Here are the decorated trees in the order you will stumble upon them: