i am now at level 50. i copy these info from somewhere . maybe can help u a bit.

"First of all, get that Charisma perk that lets you link between settlements- Get one settlement to have abundant of food and water, not too much but more than that the settlement needs- Link this settlement to small settlement, e.g. Red Rocket. Make sure there's NOTHING build firsthand here and scrap everything- Build one bed, one turret with 5 defense, and one Bar (Food and Drinks trader level 3)- Send only ONE settler from any other settlement, set him/her to the bar- You need to increase the size of the settlement ( the size bar on top-left screen) to at least orange or full- Then you can just leave, no need to wait there at all- Return when it's at 95% happiness or plus, and then stay there till 100%It's proven for me, but you guys can try."