The key is getting and using The Creator special summon effect when Slifer is in the graveyard discarded or sacrificed. Make sure you are playing level 10 Tea to use the Possessed Soul effect to sacrifice and clear the field before you deck out. Hope this helps!
No problem! I'm sure there's faster builds better but you get might get lucky to summon from the graveyard a few times if you get creator and Slifer quickly. There's a glitch if you try to special summon Slifer by The Creator when Ras Desciple is on your field.