A piece of a cardboard box, like one that you receive when you get packages in the mail, will work to make DIY Yarn Hearts. You want something a little thicker, like corrugated cardboard, rather than a cardboard cereal box. Cardboard that's thin won't really hold up in this craft. It will bend and flex and this craft will be a lot harder than it needs to be. But, if all you have is thin cardboard, you can make it work. Just be really careful when you're wrapping it with yarn.