The most difficult part would be making the heads.
You have several several options:
- making it with paper mache (you'd use a balloon as a base)
- styrofoam carving
- making a base with a chicken wire and then... again paper mache or foam
Check these homemade costume examples for details on using different techniques:
Lego Minifigure costume (foamcore)
http://www.costume-works.com/lego_minifigure_bad_guy.html
Dragon costume (foam):
http://www.costume-works.com/dragon.html
Matryoshka costume (chicken wire)
http://www.costume-works.com/matryoshka.html
Charlie Brown costume (paper mache)
http://www.costume-works.com/charlie_brown1.html
Unicorn (home insulation foam carving)
http://www.costume-works.com/unicorn1.html
Jack Skellington (styrofoam)
http://www.costume-works.com/jack_skellington.html
Hope this is enough to give you ideas :)
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For these particular costumes what you selected, I think you need to make fake round heads... If you just paint eyes/faces - you may have to explain people what your costumes are :)