Unless a cat is sleeping, it's practically impossible to get them to sit and pose for you -- the cat is far more likely to want to try and play with your moving paintbrush! So spend some time trying to get a good reference photo (or a collection) that you're going to use as the inspiration for your cat painting.
Put the photo up on a nearby wall, or pin it to your easel, so you can quickly and easily check something, such as exactly where a band of colour goes.
The cat in this photo is called Scruffy. When she first came to live with us from animal rescue we called her Fluffy (I know, it's hardly original), but she quickly revealed that this was too much of a ladylike name for her character, so it evolved to Scruffy. The photo was taken when she was sitting on the roof of our car.