Use the point of the craft knife to carve an outline of the bottom of the heart down one layer of the skin to the white part. Do not cut completely through or the handle will detach.
Carve the centers of the hearts.
Slice inward around the rim of the watermelon to remove the flesh.
Trim the flesh from the underside of the heart handle, leaving the handle at least 1 inch thick. If you trim it too closely, it will wilt and sag.
Wipe off the marker outlines with a clean cloth.