Cement (gluing) is optional. The mannequin will stay together well if you take the time to push and lightly tap them into the joints with a hammer. If you want to take it apart, don't glue it, but be warned that the parts near the base have a tendency to rotate forward and backward once weighted so gluing these bits may be advantageous. Even with the base glued you'll still be able to take it apart for easy storage.
Use wadded up newspapers, light pillows, or polyester filter floss to add bulk and volume around body and inside mask and shoulders to give it some volume.
Dress with your favorite costume components and you're done!
for contributed photos, more detailed instructions, variations on this theme, PVC resources, and links to other PVC mannequins. Thanks for reading!