Begin by roughly shading an area. You do not need to be smooth.
Smooth it out with cotton pads or tissue paper.
Draw lines across the shaded area. Switch between using the sharp point and flat blade of the pencil to get some variety in the shape and strength of the lines. Also try and make the lines parallel to one another and make them fairly long or else the wood will start looking like animal fur.
Get an eraser and drag it over the drawing in the same manner that you used to draw the lines in the previous step. Again, try and keep the erased lines parallel with the drawn ones. I could have done a better job in that department, but I can fix that later on.
Repeat step 3 by drawing more lines over the wood, but do not completely cover the erased sections as they are supposed to serve as highlights. If the erased sections were a bit crooked like mine were, you can correct that now by reshaping them.
Now you must add finer lines and maybe reduce the size of some of the highlights if the wood is looking too patchy. The finer lines add detail to the wood texture. I also add darker, more defined lines in places to give a bit more tonal variance to the wood.