Step 8: Stuff the inside and or Decorate (Optional)
The form should be stiff enough to not collapse, but you can stuff the inside if you want to make it sturdier.
Things that should work include: packing peanuts (in a bag, unless you seal off the bottom and armholes, fill through the neck, then seal), polyfil, old pillows, packing foam... see the next step for details on how I stuffed and mounted mine.
I am using a layer of insulation spray foam to coat about 1" on the inside (so pins will hold better). If you do this, just spray one section at a time, let it dry and set for a few hours, then flip the form over and do the other section.
You could also shellac the outside if you wanted to seal it, or decoupage, collage, paint, spraypaint, whatever... its not necessary, but if you are using this as a display mannequin decorating it could be really nice. I might decoupage some newspaper and lace onto mine.
Alternately you can stretch and attach a sweater/ knit shirt over the paper form to create a better surface for pinning fabric.