When you first enter Judgement Hall, make sure to save before you get judged by Sans. After his speech, you must quit the game, then reload the save you had made.

After Sans's judgement, he will give you some additional dialog asking you to reset the game again. Do this a couple of times, and he will eventually give you a key. You can reset the game again for some additional dialog from him.

After getting Sans's key, you must head back to his house in Snowdin. Once you get there, you then need to interact with Sans's bedroom (the one with flames underneath it.)

You will then be put into a pitch black room. Walk around the space for a while until you suddenly vanish, in which Papyrus will suddenly speak, and reveal that you had been running on a treadmill the entire time.

Once he is done speaking, interact with the cardboard box dresser and you will find another key.

Once you get the 2nd key, you must then go behind Sans's house, past the mail boxes, and finally interact behind it.