Here's a simple Kirby costume we made for Halloween.
Step 1: Create the body
We bought the biggest and cheapest yoga ball we could find.
Overinflated it so it was big enough that my son could fit inside.
Then we paper mached the whole thing with 3-4 layers. I found it was easier to just rub the paste all over then stick large sheets of newspaper on top the rub more paste on that.
We had it sitting on a bucket that served as the opening in the bottom.
Once it was completely dry just deflate and remove the yoga ball.