To earn every Tattoo (Tatau), you'll need to earn XP by playing through the game. XP can be gained by completing missions, doing side tasks like the WANTED Dead/Path of the Hunter, taking over Outposts or Crafting. For each time you level up, as shown in the Skill menu, you'll receive 1 Skill Point to use.

Not all Skills will be available to you from the start and some will only unlock once you've progressed in the main story or completed certain objectives. I'll list below the few you should really try and complete before you advance too far into the story.

Heron Tree

Dual Death From Above
Allows you to kill two people simultaneously from above for 7.5 times normal experience per kill.
- Requires you to perform one Death from Above kill

Dual Death From Below
Allows you to kill two people simultaneously from below for 7.5 times normal experience per kill.
- Requires you to perform one Death from Below kill

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Spider Tree

Ninja Step
You make less noise while running and sprinting.
Requires you to liberate 2 outposts without being detected

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The 1 in the Ninja Tree will be impossible to unlock if you've taken over all Outposts in the game but have not completed the Skills objective. So make sure you get this done as soon as possible as they get harder the further you get into the game. The other 2 in the Heron Tree aren't as hard or missable, but may be extremely hard to come across later on in the game again when all the Outposts are gone.

Missing any of these 3 will result in you having to start the game again and playthrough quite far for the required Skills.