Unlike joining the Institute, joining Brotherhood of Steel does not require you to complete anything whatsoever and you’re free to join the faction whenever you see fit!
How to Join Brotherhood of Steel
In order to begin, all you need to do is to give Military Frequency AF-95 a quick listen followed by paying the Cambridge Police Station a quick visit.
Alternatively, you can simply observe a gigantic airship flying above and appearing at the Boston Airport. Doing any of these aforementioned things will allow you to join the Brotherhood of Steel faction.
Similar to other factions, you will be able to pursue optional missions offered by the faction for as long as you prefer, however, do note that if you continue to complete optional missions offered by Brotherhood of Steel, you will negatively impact your relationships with the Institute, the Railroad, and the Minutemen.
This is all you need to know in order to understand how to join the Brotherhood of Steel. If there is something else you would like to know, make sure to let us know in the comments section below!