Assassin's Creed Valhalla

Your settlement level is increased two ways: building the various buildings around your settlement, and allying with certain territories. Note that, while building a building always increases your settlement level meter, you only get an increase to the meter from allying with certain territories. Specifically, when looking at the alliance map and hovering over a territory, you'll be shown the reward for allying with it. If the reward is a blue square with a house icon, completing that questline will increse your settlement level meter.
 
In order to reach level 6 for your settlement, you'll need to complete all territory pledges that give settlement increases. You'll also need to build every building in your settlement. Note that there are three buildings - cattle farm, grain farm, and fowl farm - that can be "built" multiple times. You need to build all three of them four times each as well. In order to build this many buildings, you'll need to complete all raids on the map, in England. Doing so will provide you with all of the Raw Materies in the game, which you'll need all of in order to build all of the buildings. Raids will also provide you with Supplies, but unfortunately you will NOT get enough Supplies from all of the raids in order to build every building. You'll likely be upwards of 4000 Supplies short of what you need.
 
The only other way to get Supplies is from the regular lootable chests. However, as you may have noticed, some chests contain leather and iron, and some chests contain Supplies. Thankfully, it isn't completely random. The chests that contain Supplies are those chests that are found in civilian areas only. First and foremost, any regular chests found in monasteries and abbeys that you raid should absolutely be looted. These will always have Supplies, so never leave a completed raid without making sure you get all of the regular chests too. Check your map for the fainter yellow dots for the regular chests. With those all done, unless you were very meticulous about collecting chests everywhere, you'll now need to hunt down chests with Supplies in them. As I mentioned, only civilian area chests have Supplies. This means that any place labeled "garrison" or "fort" or any restricted areas with guards will NOT have supplies. Those chests will always have iron and leather. The same applies to any chests that are located in tunnels underground, and any located under water. Distrust areas, on the other hand, are perfectly fine.

To start, go to Lunden and Jorvik, two cities, and loot the chests there. Remember, especially in Lunden, that if it's a guarded restricted area, don't bother with chests in those areas. Likewise if you see any barracks or garrison in/around the city. Stick to the civilian areas. Once those are done, your next best places to go are the little towns with port fast travels (the anchor icon), such as Ledecestre, Grantebridge, Northwic, etc. Remember, don't waste time with chests underground or under water. Also, specifically do NOT waste your time with Wincestre (despite it being a city, every chest has only iron and leather), Donecaestre (not civilian; all chests are iron and leather), or any other encampments.
 
Once you have enough Supplies, you can finally build all buildings. You can get an idea of how many more Supplies you need based on how many more Raw Materials you have left. After doing all raids, building every building will use up every Raw Material, so if you have more than zero Raw Materials, you still have buildings left to build. Once they're all built, you'll finally reach settlement level 6 and unlock this achievement.