First collect all of the Jellyfish people and beat the game once. There already exists a great guide on how to find the Jellyfish people here:

How To Find All Jellyfish

Go and dream in your bed and go to the room where you go for the ending.





You will find yourself back in your room. Go through the door to outside again. You will be in a hallway again and just like the normal ending everything will go black. Go past the elevator and do not go inside. Go to the second to last door in this hallway. Inside you will find a room with a bunch of boxes and three venus symbols (♀). Touch the symbols they will start glowing.



Go up the elevator and head down the hall to a door at the very end.



Enter this door, you will find yourself on the roof in a massive storm.



Head to the left and you will find another trio of venus symbols. Touch them and you will teleport onto a spaceship.



Once on the ship go up to Masada Sensei and talk to him. He will play a song. Afterward on an Xbox Controller Press AYX in that order. He will play the song again repeat the combination AYX. Head through the door and lay down.

Congratulations you got the ending!

Once on the roof walk down to a raised square in the lower south part of the roof.

Jump on top of that square. Afterwards jump from that square to the area on the roof.

It may take a few attempts. Here is a gif of how to jump:



If you are having trouble jumping try putting the game in windowed mode and lowering the graphics quality to their lowest settings. This should help.

Don't be dicouraged if you don't make it on your first try, it typically takes a few attempts.