Along the bridge connecting the resort to the Core, Undyne will call you asking you to deliver a letter for her, telling you she'll be waiting outside Papyrus' house where she'll give you the letter upon talking to her. (note: this event will only happen if you have completed the Neutral Route, though you can still complete the True Pacifist Route on your first playthrough - you can continue the game, finish the Neutral Route, reload your save, and then backtrack, where you should recieve the phone call.) (If you have already reached this bridge and have completed a neutral run when you go on the "date" with Undyne, you will get the letter at Undyne's House, or Papyrus' house if your inventory is full.)
Put the letter under the door of Alphys' Lab and complete the "date" with her.
Return to Alphys' Lab and enter the "bathroom" to access the True Lab.
Find the four keys scattered throughout the True Lab and put them in their proper slots to open the generator room. Turn on the generator to trigger a conversation with Alphys.