Thanks a lot, means the world to me to see people enjoying the game! Ok, let me give you a nudge in the right direction :)
*** SPOILER ALERT ***
Reaching the bad ending is pretty easy to achieve. If you decide to let Cora go with Rosie, you will reach the bad ending regardless of what you do afterwards. The route to reach the good or even the neutral ending is closed at this point.
You actually need two do two consecutive things at the ritual in a timed event for the good ending. When you get the hint that you can now intervene the ritual, and based on your experience with the neutral ending this is certainly what you want to do, the first thing that needs to be done will be something that causes distraction and that buys you another turn where you can do the final thing that leads to the good ending. For the distraction part you might want to examine something that you took with you from the manor. Something which you maybe thought it has no more use. Something that actually says it might be effective in certain situations when you examine it.
But it is not only about that because there is a small cascade of events including another riddle to solve that will ultimately lead to an item which is needed to reach the good ending. I don't want to spoil the whole thing for you but try this: In Part II, when going back to the manor after you've spoken to Ysabella, get the things she advises to and do what she asks you to do in the dungeon. When Rosie asks you to take Cora with her, reject. Before heading back to the ritual, visit the Dovecote. You might stumble upon someone you didn't expect to meet there. The rest is up to you, because otherwise there would be no fun in it.
If you're really stuck again after this: there is a transscript in the game archive you've downloaded here. The transscript is a complete playthrough of the game that showcases how one can reach the good ending.