This repeatable Minutemen recruitment quest can be found by speaking with Preston Garvey. It comes from a settlement that knows the importance of expansion. They found a nearby location of value and would like you to prime the spot for a new settlement. We found that Garvey typically doesn't assign this until you have at least a half dozen other Minutemen side quests under your belt.
When you can pursue this mission, head to the randomly designated settlement. Speak to the waypoint-indicated settler for the quest details. She'll point out a nearby location and the need to rid it of all hostiles.
Head over there and wipe out whoever is occupying the place. It could be Raiders, Ghouls or even Radroaches. Whatever they are, be ready to take on at least one buffed up version of whatever enemy type you encounter.
When you've cleaned out the place, stop at the site's workshop to signify that this area is ready for settlement. Head back to the other settlement to report your success. Then speak with Preston Garvey to complete the quest.