Fun ending isnt that complex in my opinion all you have to di is collect the clues.
Viewing post in How ACCUALY to get Fun ending
First in the relax ending go to the biat and read the note and then he Tells you to find the glowing key which is in the shack and then u fight hım in which u spare hım to get the shard (sigh) in which u hibe to the Man who speaks in hands (gaster) and there u go ITS more like rng and tries than clues but still. (Just realized i had a stroke while writing this)