Instructables user BendikM made his girlfriend a Supernatural themed calendar for his girlfriend. Part of the gift was three replica props from the series, including the First Blade. To duplicate the important weapon seen in the television series, BendikM used paper clay and paint. He used photos from the web to make a template for the knife and scaled it to fit his hand, and then the template was transferred to thick wood before being covered with paper clay. You can watch the build come together in the above video.
Read about the process at Instructables.