The first thing you want to do is go to the shop and buy the Broken Relic. This will cost you 1 coin. If you don't have a single coin you can go behind the shop and walk left to go behind the buildings where you will find a coin at the end.
Now equip the relic and talk to the 3 climbers from 1st place to 3rd. Each one of these NPCs will mention a direction when speaking (this is random for each player, in my case it was UP, LEFTDOWN & RIGHT).
Finally go to the graveyard and press A on the gravestones in the order that the climbers mentioned (always starting from the middle gravestone e.g. in my case UP = the top stone, LEFT DOWN = the bottom left & RIGHT = the stone to the right of the center). If done correctly a beam of light should shine down on the middle gravestone allowing you to fight the secret boss.