The first thing I do is to transfer the outline and key points of the image onto the wood. All the real detailing comes with the shading later.
Transferring the image is easy enough, you can use transfer paper, but sometimes the line isn't easily erased after the burning is done if it shows through the lighter areas.
I just go back to grade school and cover the back of the image with pencil, flip it over, align it, and trace.
(Tip: use a red pen, that way you'll know what you've done and that you haven't missed anything.)