On this page of the Assassin's Creed Valhalla Wrath of the Druids game guide you will find a detailed walkthrough of the Dyed in the Wool quest.
By rebuilding the trade post in Drumlish, you will gain access to Dyed Cloths - one of the main luxury goods in the Wrath of the Druids expansion pack.
Reward for completing the mission: 1,700 XP.
Investigate the Trade Post
Once you have cleared the trade post, read the note attached to the tree in the location marked in the image above. You will learn where to look for the document you'll need to rebuild this trade post.
Obtain the deed
Head to the location indicated on the world map and use Odin's Sight to locate the hidden document. Stand roughly near the area pictured in the image above and shoot at the oil jug to blow up the debris. Now you can take the document and return to the trade post.
Restore the Trade Post
Return to the trade post in Drumlish, interact with the wooden plaque and confirm the construction of the new building. This will complete the Dyed in the Wool quest and you will receive 1,700 XP as a reward