Form Ranks

Form Ranks is a Minutemen main quest in Fallout 4.

Detailed walkthrough
After completing the quest Inside Job and getting Banished from the Institute, talk to Preston Garvey about how to take down the Institute. Garvey will state that the Minutemen are not strong enough to take on the Institute. They will need to recruit more settlements in order to have the manpower necessary to stand a chance.

This quest will stay active until the Sole Survivor has recruited eight settlements. If they have already recruited eight, then Preston will give 1 additional settlement quest, which the player will need to complete to advance. To recruit settlements, listen to Radio Freedom or talk to Preston to receive information about settlements that have requested aid.

Once there are eight settlements, the Castle will come under attack by the Institute, triggering Defend the Castle.