Welcome! I did this guide because I found the Conspiracy trophy the most difficult for me; namely, there are many prerequisites for going on this path that are... unstated. After searching for an explanation for my many failures and finding none, I consider it my duty to help others who may be in a similar situation.

But enough of that, here's what you need to do!

Firstly, go on the Ellen path. There are plenty of opportunities for Ellen points, so don't worry too much about that.

For this trophy, she must NOT be treasurer, so either run for treasurer or avoid saying "wait and see". For an easy point, choose "say you like him", instead.

You CANNOT be a member of the secret society, so if you get an mental invitation to the gym at night, reject it. Whether you join or not, you need to be out of your room on the 4th of December, so schedule either gym or a class for then.

On the 2nd of December, Ellen should be pissed at Grabiner. You want to get her in trouble, so tell her "It's not fair".

If things have worked out so far, you should get your bra back on December 5th.

If you've got at least one Ellen point, then when you think of her on the 20th, she'll be thinking of you.

Now comes the third thing not to do: you need to stay friends with her without entering into a romance. This should be accomplished by NOT sending a Valentine to her.

The rest should play itself out, more or less, as long as you stay committed to Ellen's path. (i.e. no marriage or Damien)

For best results, have Smarts over 40 by March. You'll get better insight into what she's planning, and will have the option to become Architect of her Grand Scheme if you choose "Wait a minute..."

Thank you for reading, let me know if I've made any mistakes!

DO:

• Pursue Ellen's friendship
  
• Encourage a freakout at Grabiner on Dec 2nd
  
• Leave the room on Dec 4th

DON'T:

• Let her run for Treasurer
  
• Send her a Valentine
  
• Join the revenge club

As comments below have stated, you can send her a Valentine and still be brought in on her scheme. I could swear that I sent her a Valentine and didn't get the achievement, then reloaded the same file, didn't send her a Valentine, and did get the achievement. So in retrospect, I have no idea what could explain that other than misremembering. In any case, I'll leave it up to you to decide whether you want to pursue her romance or not.

Likewise, I tried a playthrough where I stayed in my room on December 4th, and that kept me from getting her ending, whereas the same playthrough (different save) I left, and I did get her ending. Apparently you can get her ending either way, but without knowing how, there's not much I can say. Sorry, hope you have luck with whatever you choose. :/

Thank you all for reading and commenting, I hope this can continue getting ever closer to the truth.