How do I make a cake board look like wood?

There are tools for that...
http://www.google.com/#hl=en&output=search&sclient=psy-ab&q=wood+grain+paint+tool&oq=woodgrain+paint+tool&gs_l=hp.1.0.0i10j0i5i10.4167.14821.1.19351.20.20.0.0.0.0.198.1710.16j4.20.0.les%3B..0.0...1c.e80D4jh-GWY&psj=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=f0823261d2025598&biw=1272&bih=649
Cover with icing and drag the tool through it,
or,
cover with fondant and use an impression mat or score your own woodgrain freestyle,
or,
cover with woodgrain Contact Paper.

I have covered a board with woodgrain scrapbooking paper, then with clear contact paper labled as food safe...or you could use something like this:
http://www.cableorganizer.com/decorative-contact-paper/