I found several instructions online using water and granulated sugar:
Using a jar and foil, form your ghost's shape.
In a large bowl, mix 1 cup sugar and 2 cups water thoroughly until the sugar is dissolved.
Drown pieces of the cheesecloth in the bowl. Ring out excess water and drape over your ghost form.
Let dry 24 hours.
In theory, it should be stiff. Mine wasn't. The sugar had failed me. So I tried applying some fabric stiffener to vulnerable areas.
Twenty-four hours later, it was stiff, but still kinda flimsy. Maybe I didn't use enough stiffener, but next time, I'll try corn starch.