Okay, my god, I've spent the entire day tryna get the 'Truest ending' and I think I've figured it out. I just thought I'd put this here in case anyone else is wondering cos I for sure was looking for a guide too lmao.
SO, MAJOR SPOILERS AHEAD! (don't read this if you wanna figure it out on your own).
You must obtain all 8 endings first being Sans, Papyrus, Undyne and the True routes - each having a 'neutral' and 'genocide' ending.
I believe to get a neutral ending, you must not investigate any of the items and disregard them, e.g. don't talk to Sans, Asgore, and Muffet and choose the option, 'stay determined'. I also was being a little rude brat with my dialogue options but idk if that affects the route. At the very end, Sona should take control of the game and close the application.
To get a 'genocide' ending, I believe you must investigate all the items and be 'nice' and such with your dialogue options (at least that's what I did). At the end, Sona will ask the character in which you have chosen (sans, papyrus or undyne) to kill her and they will refuse. Then the 'player' will use this as an opportunity to kill Sona and then end the game. (Ik, sounds confusing, but you'll get there)
The 'true' pathways just involves not directing a particular character to befriend/date Sona and then just doing going over the steps for a 'neutral' and 'genocide' ending.
You will know when you've achieved all endings when Chara asks you to help them save their sister at the very end of your 8th route. The truest ending is where EVERYONE survives.
Okay, that's it I think. I'm gonna try this again later to see if I'm right cos I literally was all over the place and the process of finding all the routes was so tedious that my mind is kinda fuzzy, so I can't be sure that I'm completely correct with this guide.
Despite being tedious, the game was fun and well-written! I enjoyed it. Thanks!