Hi! Sorry about that, I added some checkpoints in the latest update, here's a guide for all Nadia endings:
Ending 1: Finish the Max ending path in "Nadia Past" story, it will trigger after the dinner scene
Ending 2: Finish the Max ending path but decide to not forgive Celeste
Now for the Ruby endings, start Ruby from scratch, after the first date tell her you enjoyed it and "Yes, both of you are amazing" and continue there, this will trigger the normal Nadia story but opening the poly path
Ending 3: After finishing "Nadia Past" picking Brooke, continue the story on Nadia chat, until the point you have the possibility to talk ro Ruby, talk with Ruby and she will decide to go see Nadia, then the special event "Nadia and Ruby" will appear, at some point you can also decide to bring Nadia to see Celeste, you will have to talk with Brooke if you decide to take her and then accept Ruby in your life at the end
Ending 4: Same as Ending 3, but you don't mention visiting Celeste
Ending 5: Same as Ending 3 and 4, but you don't accept Ruby at the end
Ending 6: Bad ending, it happens when you pick Brooke path and then don't contact Ruby