First lay out your newspaper on the table so the paint doesn't get everywhere and put an old t-shirt over the kids clothes if you don't want them messed up. I used white cardstock for my daughter to decorate with shell prints. She painted the shell and then pressed it onto the page. It's good to have a mixture of sizes so you can get a really nice variation on the paper.
Here is a picture of the finished shell prints. I think I might use mine as a scrapbook page.