Among the many wonderful characters created by Dr. Theodore Seuss Geisel, the Cat in the Hat remains a favourite of young readers. The wily cat is a fun character and a great choice for a Halloween costume. It's not the easiest costume choice, but it's far from the hardest.
Get the hat. You won't find this extra-tall red and white hat in shops (except possibly in some novelty fashion shop), but you can make your own if you have time. Go low-tech with cloth strips or even magic markers.
Get the bow-tie. You can find one of these at any Salvation Army shop, or you can make your own from a red sash, mimicking Seuss's drawing.
Make up a cat face. Some black make-up and fake whiskers should do the trick.
Get cat fur. This is optional, as it takes a lot of effort. One easy way to add fur to your costume is to get any kind of square cut of furry material and just wrap it around the shoulders. It's not form-fitting, but it should look okay. Some die-hard cat fans can find skilled seamsters/seamstresses who will put together actual furry costumes with arms, legs, but that's above and beyond the general standard of cat costume creation.
Get props. Seuss's cat loves to balance household items in funny and chaotic ways. Your cat costume could include common items like books and umbrellas that the cat drags with him/her around the neighbourhood.
A great visual addition to a Cat in the Hat costume is the "pink" that Seuss's cat leaves behind on everything he touches. Any kind of bright pink plastic material will do.