No problem! I'll be more happy to do so! So assuming you've already gotten the other endings, you now know how to go back in the cave to meet with sans.
Here's some key points after coming back:
*When you wake up from the strange feeling of being watched from the dark, change out your thoughts appropriately and also the next scene right after- something close of calling out San's name and that you wish he would have stayed or something.
*When Sans invites you to his house, I dunno if this is important but imma just gonna share it, you need to change your thoughts into something close of wanting to help him from his trash like house (Papyrus would be so happy!)
*The scene will then appear when 'you' brought up the time when Sans was watching you sleep- which may lead to sleeping together as long as you pursued on your asking until he agrees on it.
*When the night terror comes into the scene, you will need to change your thoughts very appropriate after Sans comes backs to his senses, something close to saying 'I love you' maybe...?
*When the confessing subject comes back, this is where my headache came to root- you will have three scenes and in those, there are 3-4 choices each. Pick the wrong ones and well, the subject has been dropped and no more romance route for you my friend. Here is the order of what you should choose: 2, 4, 3
And there my friend will be a smooth sailing journey from no on for the romance ending! Enjoy the bonus >w<//