To start the true pacifist run, you need to encounter Flowey. Toriel saves you, and guides you through the Ruins. Then you get to the Dummy, go to ACT and use the option: Talk.
Then you encounter a Froggit, go to ACT and use: Compliment, do the same for every Froggit you find. If you find a Whimsun, simply go to MERCY and spare it.
Encountered a Loox? Spare it and wait if it says: Please don't pick on me. or: Pick on me. If it says one of those, go to ACT and do: Don't pick on or Pick on.
Do this for the whole way through. Then get to the Toriel fight and keep sparing her until she lets you exit the RUINS.

Simply go through Snowdin and find Doggo, go to ACT and use Pet once, then go to MERCY and use Spare, continue and do papyrus' puzzles and you should find Lesser Dog, simple go to ACT and use Pet, then go to MERCY and use Spare. Then you should encounter Dogamy and Dogaressa. Go to ACT and do Roll Around, then do ACT again and go to Re-sniff, Then ACT again and do Pet. Do this for both of them, then move on. Then do the puzzle and go on to the Greater Dog battle, simply do this: Go to ACT and use Beckon, then go to ACT and use Pet, then Pet, then Play, then go to ACT and Pet. Now try and spare him. Advance to Snowdin Town, advance to the next area and just spare Papyrus over and over. Then advance

Welcome to Waterfall, it's a more dangerous but calmer area. You'll encounter a person with a spear, Undyne, a person who is somewhat relentless to humans. So we'll need to be careful around her. So the main way to start is get past your first encounter with her and advance to the next area. Okay so we'll come back to Waterfall later! I forgot a part of Snowdin.

So return to Papyrus' house after the fight and talk to him, then go into his house and go to his room, a question will be asked. Make sure you respond with [Yes] or you'll mess up the whole thing. Then you hang out with him, etc.

So we found Undyne and all so now we can move on and then we find Monster Kid. Now just move on to the next area and solve the flower bridge puzzles. Congratulations on getting to 27% in the game! Now advance through the place and encounter Undyne again and then get past her, keep on going of course... Undyne shows up AGAIN and then you fight her later. Then we move on!

Welcome to Hotland, probably the biggest area in the game. Get to the lab, find Alphys, meet Mettaton, go forward and do all the puzzles that are impossible to lose on, go on Mettaton's cooking show where you nearly get killed, and then proceed through normally until you meet Mettaton for the third time, and you need to (i think) defuse all the bombs. Now you'll go forward, up the elevator, and play up to where you find 2 puzzles. Do both of them and then proceed through the door. Do all that and then walk through every room, and feel free to talk to everyone, because that really doesn't affect anything. Now go through the fight with Muffet and proceed to the CORE.

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Welcome to the end of your adventure, all you need to do is fight Asgore, he breaks the SPARE button so now you can only use FIGHT as talking won't help you. Keep fighting.

Then spare Asgore and flowey will kill him, Flowey closes the game and now you MUST reopen the game, continue and talk to him more, and engage the Omega Flowey battle.
Now survive for long enough and fight the SOULs. then they will betray Flowey and you can deal massive damage to Flowey. Take him to low HP so he's at 0 then he tries to kill you and then you beat him and then you must reopen the game again and then go back to Papyrus's house and get Undyne's Letter, deliver it to Alphys, then "Date" with Alphys and then go to the lab again and go in the "bathroom"

Welcome to the True Lab, fight the Amalgamates and be on your way, find the keys, put them in their locks, and enter the center door leading to the end.

It's time, time to leave the Underground. Don't worry about dodging Asriel that much, you'll just come back in after going through the message (But it refused.). Now just go through the fight how you think you should, SAVE your friends, and be on your way out of the UNDERGROUND with everyone.