The Outer Worlds

This achievement is a hidden fourth ending.

At any point before collecting Olivia Ambrose's Journal for Minnie Ambrose, kill Minnie at the Ambrose Manor. Return to your ship and you will be contacted by Spacer's Choice and Lucien Bancroft. He will ask you to collect the journal for him instead. Play out the rest of the story as you normally would. During the last mission Side with Lucien and turn on the Reactor. Lucien will then contact you again asking you to drop off the journal at Byzantium. Once you drop off the journal and talk to his contact, the achievement should pop.

You can reload a save to do this but make sure you have not collected the journal yet, the last step of the quest "The Red Tape of Command". If you have the journal Minnie will contact you and the normal endings will play out. Even if you kill Minnie and activate the reactor, the quest will auto complete and Minnie will respawn at the manor as if nothing happened. In anycase, Lucien will not contact you and the achievement unobtainable.

Here is a quick video guide on how to get and complete this quest line:

The achievement unlocks when you deliver Olivia’s journal to the dead drop location on Byzantium and conclude the dialog with the contact standing nearby. Although this is simple enough, there are some complex preconditions beforehand.

Following the story, once you use the terminal in the CHEM Lab and Override Synthesis & Manufactory Center Lockdown, you complete the Experiment Perilous main mission and start the main quest Red Tape of Command. This is a branch point. To keep all your options open, make a hard save at this time and do not overwrite it.

Note: If you are going for the best ending in the full game, you will want to complete the game having unlocked the Family Reunion Peril on Gorgon DLC ending. The full game's ending is heavily influenced by how you concluded the DLC. The Postman DLC ending is one of the bad Gorgon endings, perhaps the worst. Accordingly, my recommendation is to do it first and, in the process, unlock this achievement (and Greenlight Protocol). Afterwards, load up the insurance save and knock out the remaining achievements.

With the insurance save in hand, this is the roadmap to the Postman DLC ending and this achievement:

1. Go to the Ambrose manor and kill Minnie
2. On your ship, ADA patches you through to Lucien. Agree to get him Olivia’s journal.
3. Go to the SMC, get the journal, and return to your ship
4. ADA patches you through to the interested parties, concluding The Red Tape of Command
5. The Ambrose Intersection main mission is automatically initiated with multiple options
6. Side with Lucien during the three-way conversation
7. Return to the SMC and push Olivia into the reactor
8. Turn on the reactor and return to your ship
9. ADA patches you through to Lucien who instructs you where to deliver the journal
10. Go to Byzantium and place the journal in the dead drop, a mailbox
11. Turn around and talk to Simon Rentoe
12. The achievement pops upon conclusion of the conversation

Since the Postman path requires you to restore power to the Adrena-Time synthesizer, if you have not already done so, you will unlock the following achievement when you powered the reactor: