The image MUST be printed using a toner-based printer or copier.
I am using a craft/quilting iron, but you can use a full-size iron instead. Turn it on, and set it to a high heat.
Place the image, face down on the carving material.
Cover any "exposed" carving material with a blank piece of paper to make sure that the iron doesn't come into direct contact with the material.
Place iron on top of the paper and 'iron' the paper, being careful not to burn it. The paper should stick to the surface of the carving material.
Peel off protective paper, and then gently peel back the paper with the image. If it isn't dark enough, lay it back down again, and iron again.
This is the finished image. It is a bit darker than the photo portrays, but it is still difficult to see the detail.