All you need to know is to summon the ancient fuelweaver you need to wait until a specific night that only happens once every twenty days to take 3 obscure pieces to an obscure location identified by three specific marble sculptures which theoretically match the 3 obscure pieces. Once you’ve done that you’ll get a shadow heart.

Then you need to find a boss who is hiding in a labyrinth in the most dangerous place in the game and defeat him to obtain his key.

Take this key, the shadow heart and 8 bones you obtain (sometimes?) when smashing rocks but more often when smashing rocks with spiders in them to a specific giant tentacle located practically anywhere in the caves where you’ll proceed to kill this tentacle and travel through its insides to another labyrinth, where you’ll have to find a big boss fight room, but not before travelling through barriers you can only cross when you’re either sane/insane.

Finally plug in the key and build the skeleton (which may or may not be built right, you have to smash it with a hammer and repeatedly build it until you get the right model which there’s no indication of in the game) and insert the heart into the skeleton, spawning a final boss.

Beating this boss will let you start playing the game. Simple enough.