Here are my versions of homemade Spongebob and Patrick costumes. Very easy to make both!
Spongebob is basically a giant box with a poster taped on the front. I painted the back yellow, added the shoes to match, and am wearing a yellow shirt.
Patrick was a little more complicated. He is made 100% out of felt (very cheap to make). Start with the body. Model it kind of like a sweat shirt, sew, and stuff with pillows. Easy. The head was a bit trickier, but still very do-able. With a hot-glue-gun cut out the shapes for the eyes and mouth and sew onto a simple cone-style head.