Well, it could be in various places depending on the cartoon. Maybe it's in a house, a garden, or even a magical world. Hard to say for sure without more context.
It's quite simple. Just draw the cat and the bag first. Then, show the cat entering the bag in a gentle way, maybe with a little coaxing or a treat as an incentive.
First, you need to have a clear idea of the cat's appearance and personality. Then, start sketching the basic shape and features. Add details like fur texture and expressions.