For this project a lot of water would be used, so make sure your watercolor paper is at least 140 lb (300gsm) of density. The more dense is paper the less risk of bulging and paints going through where you don’t want it.
Then proceed with random curves, take a template with circles and draw different sized ones with the white pencil.
You also can grab a ruler and make black outlines with the pen. And as an option – put it under the matting frame.