This achievement is obtained by reaching
one of the four faction endings for the main questline of the game for the first time.
I made a chart that shows how to obtain
all the achievements with only one save / reload, and also the best point to save in order to obtain all 4 endings (not necessary for achievements).
Scroll down for a more detailed explanation (inside the Spoiler Tag, as it is an awful lot of text).
For starters, keep in mind a thing: having
high charisma will help you when you'll have to balance several factions at once. I recommend having at least 6-7 points of charisma, and using "charisma-enhancer" gear like Glasses, Hats or Suits, or even using, when neccessary, beer or some drugs as Grape Mentats or Day Tripper that give charisma points for a few minutes.
Open this thing for the full explanation:
*** Spoiler - click to reveal ***START MAIN MISSIONS
As soon as you're out in the Commonwealth, you'll be able to explore freely. Follow the main quest for a while, and, after completing the quest "Reunions" the Brotherhood of Steel will be available. That moment, or a little later (after "Shattered", for example), is a good time to join the available factions and advance on their questlines. I recommend advancing until your current mission is "The Glowing Sea", that way you'll have a higher level to face these missions.
JOIN THE FACTIONS
You'll probably have met the Minutemen already as you came out of the Vault. Their questline starts with "When Freedom Calls".
Once you complete the mission, make sure you complete "Sanctuary" too, as it isn't necessary for progression but it has an achievement.
"The First Step" is the mission in which you join the faction, and it unlocks a series of secondary missions related with helping settlements.
Do three of these secondary missions, and the next mission, "Taking Independence", will be available. Do it, and then the next one, "Old guns". Upon completing these, you'll have all Minutemen achievements, and you can forget of this faction for the rest of the game if you wish.
Time to join the Brotherhood. You'll find a military radio channel that will lead you into a police station, where you find some Brotherhood members. You'll have to complete a few missions until you become a member. "Reveille", "Fire Support", "Call To Arms", and, finally, "Semper Invicta".
You can do a few more quests for them, and I reccommend to do it. These are available after you complete the main mission "Reunions", and are "Shadow Of Steel", "Tour Of Duty" and "Show No Mercy".
Railroad time. You'll only be able to do a couple of missions for them at this point. These are "Road to Freedom" and "Tradecraft", in which you join the faction.
Okay, time to continue the main missions.
CONTINUE MAIN MISSIONS
After "The Glowing Sea", you'll complete "Hunter / Hunted"
And here the decisions start. In the next mission, "The Molecular Level", you'll need to
Make sure you choose the Railroad, to ensure you can access everything later.
Next mission is "Institutionalized". And here, you are offered another choice. You need to
CONTINUE WITH FACTIONS
Time to deviate from the main quests and work in the factions again. As soon as you complete "Institutionalized", start "Underground Undercover" for the Railroad, and advance in the quest until your objective is
Then, switch to Brotherhood missions. Complete "From Within", "Outside The Wire" and "Liberty Reprimed". Now "Blind Betrayal" will be available, but DON'T COMPLETE IT. If you do, Railroad quests will turn unavailable. So, let's get back to the Institute.
REACH THE DIVISION POINT
Complete "Synth Retention". Then, in "The Battle Of Bunker Hill", you'll have two optional objetives. Choose to
and, at the end of the mission
Complete the next mission, "Mankind - Redefined".
This is the DIVISION POINT where the different paths to the ending start. I believe this is a good moment to get all miscellaneous achievements done (Bobbleheads, Miscellaneous Quests, Secondary Quests, Giant Enemies, Settlement Hapiness, etc) so you can then pick tne ending you want and conserve all of the benefits of these tasks.
And here's the important point. As soon as the next mission, "Mass Fusion" asks you to make a choice, MAKE A MANUAL SAVE. I'd create 3 or 4 saves in that moment, for precaution. Write the location and time of the save in a piece of paper so you can recognise it easily later on.
And now, you'll have 2 different paths to the ending.
PATH 1: RAILROAD + INSTITUTE
In "Mass Fusion", choose
This will turn you into a permanent enemy of the Brotherhood, making their quests dissapear. But you can still work for the Institute and Railroad.
Complete "Pinned" and "Powering up" for the Institute.
As soon as you finish "Powering up", you can finally complete "Underground undercover" for the Railroad. Do so.
Continue with the railroad questline. Dependind of the secondary missions you have done for them, you'll have to do a quest named "Operation Ticonderoga", or you may jump directly to the next mission, "Precipice of War". Anyways, complete those and the next one, "Rockets' Red Glare", for the last Railroad achievement.
Stop here, because the next Railroad mission ends the main questline. Time to go back to the Institute and complete their missions (You can make another save here and go back later for the Railroad ending, if you wish). "End Of The Line" will turn the Railroad hostile, and, because you already did "Rocket's Red Glare", you'll skip "Airship Down" and jump directly to the final Institute mission, "Nuclear Family".
And if this is your fist "final mission", you'll get the achievement for completing the main storyline.
PATH 2: BROTHERHOOD
In "Mass Fusion", choose
This will turn you into a permanent enemy of the Institute, making their quests dissapear. You'll be able to work for the Railroad at this point, but we won't do that.
As soon as you finish "Mass Fusion", "Spoils Of War" will be available. Complete it (make a save here if you want to do the Minutemen ending later), and go on the next quest, "Blind Betrayal". Now we can complete it without messing the other achievements.
Why do I say that? Because, as soon as you complete this quest, the next, "Tactical thinking" will start automatically... and it will block the Railroad quests, making them unobtainable. But at this point tat doesn't matter, because we will obtain them in the other path.
After "Tactical Thinking", complete "Ad Victoriam"
And, finally, "The Nuclear Option".
Railroad and Minutemen have their own "Nuclear Option" too, but only one of those is required for the achievement. So if you don't want to end the game with the Brotherhood, reload one of the two optional saves I mentioned, and complete the game with either of those factions to get the achievement.
Hope it helps!