How does one make breasts for a Halloween costume?

Well, the easiest would be to wear a bra under the costume and stuff it with toilet paper or Kleenex or whatever to make it look full. ... if that doesn't work, and you want a customized look, you could also use paper-mache, which will allow a horror look or bizarre defects if you like, depending on how you shape the paper... and then paint it afterward to look appropriate. If you want ultra-realistic you could do a plastic mold filled with gel... but that is getting into the really expensive/hard to do side of crafts. :)

Easy: cut lemon/grapefruit in half (depends on size needed) and stuff into a bra. Or use little bags of sand.