I used two and a bit balls of Debbie Bliss Merino chunky and 5mm double pointed needles. My gauge was off so a few bits needed working out, but after figuring out the right amount to cast on it mostly ended up being things like picking up a couple less stitches for the earflaps etc. On my first try I placed the kitty ears too far back on the head so I unravelled them and moved them two centimetres forward. It was only my second try working with double pointed needles and I found it much more comfortable working with the chunky yarn. My last try was on a sock with tiny 3mm needles - maybe now I've more practice I could go back to the sock! I'd been meaning to make this hat for ages and now I've found out how easy it is I'm sure I'll make many more as presents for friends.
The pattern can also be found online here: http://kittyville.com/knit/kitty_hat.html