Follow the Minuteman quests given by Preston Garvey until he suggests capturing "The Castle"
Once you capture The Castle and
, power up the radio in the middle using the workshop. There are enough materials already at the castle to build two medium generators for the 10 power required.
Once the radio is powered up, complete the castle quest by talking to Preston Garvey again.
Fast travel away, and tune to Radio Freedom.
A few in-game days later, a message will play on radio freedom calling you to the castle.
Begin "Old Guns" by talking to Ronnie Shaw.
Make sure you haven't built too much in the castle, as it can mess with the pathfinding of NPCs during this quest that will make it take longer or softlock it until you clear obstacles.
Follow Ronnie to the chamber under The Castle.
*** Spoiler - click to reveal ***Be ready to fight Sarge, a haywire sentry bot. If you are struggling, use Power Armor and target Sarge's fusion cores using VATS. Alternatively, shutting him down using the Robotics Expert perk at rank 1 will complete the quest stage.
Ronnie *should* hack an inaccessible terminal to let you get through a door where you'll find
If she doesn't, sit down on one of the nearby chairs and wait an hour, and that should fix it.
Collect the artillery plans from the room you are allowed access to at this points, and collect spare parts. You can also scrap the damaged artillery on the walls of the Castle for most of the components for a functional one.
Build the artillery anywhere in the castle and assign a settler to it. If you haven't built a radar beacon at the Castle yet, you can assign one of the minutemen wandering around.
Test the artillery at the location marked by the quest by throwing one of the smoke grenades.
Talk to Ronnie Shaw, and then talk to Preston Garvey to complete the quest.