I found the pattern here.
I crocheted two rainbows according to the pattern for the bigger rainbow. I omitted the last two rows, because I run out of colors. This is a great project to use up stash, by the way.
Then I just sewed the rainbows together using the colors of the stripes. Be sure to leave long tails as you switch colours, so that you can use them to sew the rainbows together neatly.
I stuffed it lightly using some polyfil. Quick and easy.