Assassin's Creed Valhalla

Just like with the base game, completing all territories requires you to collect all Wealth, complete all Mysteries, and collect all Artifacts. Like always, synchronizing all of the viewpoints will add all Wealth, Mysteries, and Artifacts to your map. It then just becomes a matter of heading to each point and collecting/completing it. 
 
That being said, there are some things that could trip you up. With Wealth, the raids and treasure and gear are all straightforward, but the Books of Knowledge are all hidden behind puzzles. If you're struggling with any, refer to THIS LINK that has a full rundown on how to get each one. The only other Wealth you might struggle with is one up in a tree in Gullnamar. The only way to reach it is with the Power of the Raven. Nearby you can find large black birds you can kill and get the ability from. 
 
For Mysteries, most are extremely simple, but there are a few that you might be scratching your head on:
 

• The one with dead Bo who fell off the roof next to the grieving woman in Gullnamar requires the 'Instant Horde' upgrade for the Power of Rebirth ability on your Hugr-Rip. That upgrade revives all dead bodies around you when you use it. You'll need to use the ability three separate times on Bo to complete that event. 
• The one with Skavi and his crow Kraka that is out on a floating rock in Svaladal requires you to upgrade the Power of the Raven ability on your Hugr-Rip so that it lasts for 50 seconds. That's the only way you'll be able to fly to that floating rock (which also unlocks "Flying Fortress" when you land on it). Fly there, kill the enemies, talk to the tied up guy, get the ember nearby (use Odin Sight), then fly back to Skavi. 
• The altar in Gullnamar requires 5 regular pollock. Thankfully, they can be found in the river north-northwest of the altar, only 100m away. Just use Odin Sight to find fish and shoot the biggest ones with your bow to quickly get five regular pollock. 

 As for the Artifacts, they're the easiest since most are just sitting on the ground or on a table to pick up. The only exception are the Jotunheim Runes, which function similarly to the cursed symbols from the base game. The difference here is that you need to destroy 4-6 of them to finish each Artifact. Thankfully there is a simple trick, which is to use Odin Sight after destroying each one. This will show a trail coming out of the one you destroyed and leading to the next one, so you don't need to look around aimlessly for each one. Note that you may need to wait a few seconds after destroying each one for the trail to the next to appear. 
 
Once all four territories are fully checked off, you'll finally unlock this achievement.