Go home and unroll a good square of plastic wrap. Smear some blue food coloring all over it and roll it up into a tube. Tie both ends up into a knot or something so it won't leak and superglue one end to the belly button on the doll.

Open the ziplock bag and dump in the spaghetti sauce, seal it and turn it over so the bag covers the 'zipper' part that keeps it sealed. Superglue the other end of the plastic tube you made to the 'zipper' part of this bag. Let it dry overnight.

Find a way to transport all this and the fake blood without being seen and drive to a gas station, school or other public ladies' restroom.

Enter the bathroom and take out the fake baby, smear it liberally with the fake blood and put it in the toilet, making sure it lays head down with its back and butt in the air(or on its side). Lay the bag full of spaghetti sauce in next to it with the zipper down like before, then pour a bunch of the fake blood into the toilet and all over the toilet seat.

Get the HECK OUT OF THERE, find a place to hide and listen for the screams.