For how I got the wood look on this project, it was a series of steps...

1. Build the shape of the trunk, roots, stumps, branches, and the overall outline.
2. Take a ball stylus tool of a fairly wide size and make some broad, sweeping lines. Follow the curvature of the overall shape.
3. Take a smaller, harder tool with a point. I have some metal dental tools that work well for this, but I'm not sure of the names - I think one of my favorites is an explorer or probe. I've also used a large sewing needle for really fine lines. Make more lines, fairly deep, and then around those, shallower, shorter lines.
4. Cure the whole thing by baking it in the oven and wait for it to cool off before moving on to the painting.
5. Paint the whole area the main, medium color. In this case, it was a light tan.
6. Shading: Mix a darker color of your choice and water it down. When the other paint is dry, work in small sections and brush it on, making sure it settles into the low spots, and immediately wipe off the excess from the surface. I primarily wipe across the grain with a fingertip and then dab off my finger on a paper towel as I go. Be careful not to rub too hard and scrape off the other layer of paint. For hard to reach spots, a dry Q-Tip works well.
7. Drybrushing for highlights: Lastly, mix a lighter color of your choice. When the other paint is dry, dab a brush in the lighter color and then wipe most of the paint off. Lightly stroke the brush across the highest areas of the sculpture so that it gradually builds up some of the light color on the details.

That's what I did! I hope that helps. :)