Dodge Papyrus until he spares you, be friends with him, and go on date in his house or let him capture you three times until he gets tired and hang out with him in his house.
Flee from Undyne's fight (when your soul colour is red) and go to hot land. She will stop chasing you when her armour get so hot and she will fall on the ground. Give her water and she will go back to her home. Go to Undyne's home and you will find Papyrus. You two will enter Undyne's house and he will leave. Then you will start dating with Undyne.
Don't kill Mettaton, just dodge until the ratings get to 10000 and he will spare you.
Fight Asgore until he shows the mercy button. Flowey will kill him and tell you to hang out with your friends.
Go back until you get to the path between the core and MTT-hotel. You will receive a call from Undyne and she will tell you to go to Snowdin and deliver a message for her. When you take the message and deliver it to Alphys' lab you will swap it under the door. Alphys will think that's your message and you will date with her.
Go again to Alphys' lab and you will find a letter and a door. Then go to the real lab and finish it.
Finally go to the barrier and you will not fight Asgore again and you will get the happy ending (the true ending).
I believe you must go to Undyne's house first to befriend her, and then I think if you go around the CORE and head back to the MTT Resort, Undyne will call you and tells you to go to Snowdin to get a letter for her. After you date Alphys, you need go to Alphys's lab again, she will leave a note telling you to go to the elevator. That's all I'm gonna say, though. Don't wanna spoil.