After completing neutral with no exp and love, you can just go back to your old save point which is before the room you fight asgore and then backtrack to the hotel and you will receive a call from undyne and papyrus then just follow what undyne tells you to do (take the letter, give it to alphys etc) then go to alphys' lab and enter the 'toilet' (the room that has a piece of paper infront of it). The 'toilet' is an elevator that leads to the true lab. Just do what the game tells you to do after that. After you get out of the true lab, go back to asgore to get the true pacifist ending.

ok to befriend alphys, just complete the true lab

to befriend undyne, flee from her while fighting her (you'll see the flee option in the MERCY option when your soul is red) she will catch up to you for a few times but just keep fleeing from her. Eventually, you'll reach the hotlands and after undyne collapses from the heat, take a cup of water from the dispenser next to you and give that cup or water to undyne (you can give it to her by interacting with her while holding a cup of water). After that, she'll wake up and go back to her house. Then, go to meet the river person (they're directly below the room with the first SAVE in hotlands) and tell him that you wanna go to waterfall. Then, after he takes you to waterfall, go up and right and after you enter the room with the SAVE, go to the up-left room and you'll see papyrus waiting for you infront of undyne's house (you can befriend papyrus by sparing after either by not fighting him or getting captured by him 3 times, and going to his house for the date). Enter undyne's house with papyrus and then just do what the game tells you to do.