Step 2: Embroidering
1. Use a water soluble fabric pen to mark your design on the right side of the fabric.
2. You can make conversation hearts, cat faces, mice, whatever your imagination can come up with. Embroider your design with the floss of choice. Trial, error, and embroidering tutorials will help you immensely.
3. Once you've finished the embroidery, sew the edges with the right sides together, leaving a small unclosed area where you can turn out and stuff the heart. Stuff a little filling in and then some catnip. Top that with a little more filling so the catnip doesn't fall out. Close the rest of the edge when you are through filling.