I made this costume mostly so I could openly look at cat pictures at work all day without it being weird, but it turned out to be a really easy last minute costume that takes about an hour start to finish. I went for the classic sliced bread look for my costume, though recently, cat-breading enthusiasts have been using many new forms of bready material to push the limits of this meme (see cat pita-breading, cat tortilla-ing, cat rice-caking, and cat waffling).
Here's how I made it...
Step 1: Materials
at least 1.5'x1.5' of 2" cushion foam (fabric store)
sturdy cat ear fabric (I used fabric form a cheap witch's hat)
black eyeliner (for drawing on nose/whiskers)
hot glue gun