Summon a Goo

First of all, prepare the room for the summoning. Simply block all outside forms of light (close windows, turn lights off, etc.)

Then, sit down in roughly the middle of the room, and chant three times: 

"Belly, belly belly. Why is it so empty? Give me a creature, inside to make my belly a prominent feature."

After three times of chanting this, the creature should be inside you, a way to tell if the spell worked is if you begin to feel very hungry a few hours after casting, or if your belly begins to bulge outwards on it's own.