What age group is this costume for?
Preschool and Younger
Elementary School
Middle School
High School & Beyond

Estimated time required to make this costume:
1-2 hours

Difficulty level:
Quick and easy - no sewing required

Items you will need: (Hit your return key to start a new line)
Depends what you've got on hand or what you care to spend.

Black sweatpants and shirt, or leggings and turtleneck, work great, paired with white stretchy gloves, and an oversized red ribbon bow at the neck. An outrageous stovepipe hat in classic red and white stripes is critical to the look; it matters little whether you make it yourself or buy one, as long as you have Fun making it.

Instructions:
There's a Review from 1999 (not the original item) in the preschool section for: a Cat in a Hat costume that sounds reasonable to fabricate and wear. In that version, there's a white shirt and a black vest.
Face makeup/whiskers and the hat do much to carry the look, so look around, be creative! Tail: anything from a bathrobe belt to a purchased tail, safety-pinned on the trouser waistband. A long strip of black felt several inches wide could be "rolled" up tight, and hot glued or stitched to hold that shape, for a tail, still non-permanent and pin-able.
I'd probably carry a copy of a Seuss book around, vs. dying a platter of ham...but it still sounds very Cool and do-able!

Cruise a thrift shop or 2 and see if their Halloween goods include the classic red and white striped chapeau. These hats were a fad at fairs/carnivals for a few years. I couldn't find one at the State Fair this year, :-( but did find one at the thrift store a few days later. :-) It's made of heavy-duty felt strips sewn together, with a felt brim. I plan to hand-wash it before wearing it...in the rain, or in a boat, or in a train, here and there, or Anywhere!

Other information to consider:
Black isn't the best night costume, so some fluorescent accessories, reflective tape on the back, a flashlight, etc., can do much to help in the Safety department.