This is extremely challenging and takes nearly sixty hours to do. (That is, if you save it after millions of failures.) In order to do this, you must complete everything in Jiminy's Journal (Defeat Sephiroth, Gather every treasure, etc.) and complete every Gummi Route 100%. After doing this, go to The World That Never Was (The Altar of Naught) and go through the door. After defeating Xemnas, watch the movie that follows after the credits.