There are four settlements in Far Harbor. For this achievement you need to unlock three of them.
One is the Longfellow's cabin. It will get unlocked after the main quest "Walk in the Park." Just talk to Old Longfellow after the quest.
Then there's Dalton Farm. You unlock it during the quest chain "The Blood Tide" by Cassie Dalton. After the second quest Cassie tells you that the farm can be used as a settlement now.
National Park visitor's center is unlocked during Mitch's quest "The Hold Out". To unlock the quest, you need to complete "Living on the Edge" (main quest) and "Rite of Passage" to get access to "The Changing Tide". Talk to Mitch in the Last Plank to get his quest. Go to the Visitor's Center, talk to Uncle Ken and the Workshop gets unlocked immediately.
The fourth and last settlement is the Echo Lake Lumber also need the quest "The Changing Tide". Access the quest given by Small Bertha. After you've completed the quest, you'll receive access to the workshop.