Building up the Sanctuary settlement is one of the first miscellaneous quests you'll come across in Fallout 4. It's an introduction to the dynamic crafting system that Bethesda's implemented into Fallout 4. Unfortunately, it can be a little confusing and the instructions aren't exactly helpful.
While you might learn how to properly plant crops, set up defenses and assign characters to those crops and defenses, you might not learn how to properly set up a supply line. This is a crucial element to constructing thriving settlements.
To get started, you'll need to invest a point into the Charisma tree into a perk called Local Leader. Local Leader will allow you to send caravans from one settlement to another. Here's why these are helpful.
Caravans will connect settlements together. And if you connect settlements together, they'll share resources stored in the Workshop.
You'll need to enter Workshop mode, highlight a settler and pick Supply Line. It should be at the bottom. Once the settler becomes "Provisioner" you'll be able to share Workshop resources.
This is going to be absolutely crucial if you're trying to build something and the resource you need is stored in another workshop across the map. It has saved me a ton of time and it will save you a ton of time as well.