It's pretty easy. I do half white fondant and half brown (close to whatever color you want the wood to be). Roll each into a large log and then twist them together a few times (like your wringing water our of a towel). Roll out the fondant. You can either cover the cake with it or cut it into strips to be laid on the cake.
Once the cake is covered with it, use a knife to score it so that it looks like you have individual boards. You can rough it up a bit too if you want it to look weathered. Then pour a little clear alcohol into a container with a little brown food coloring. Paint in one direction over the fondant. This will give the wood grain look. HTH.
Some people get a brand new wood grainer from Lowe's and use that to paint the stain on as well.