Begin by shading a base layer for the wood texture. For this type of wood texture don’t make this base layer too dark or else you won’t be able to see the wood grain details.
You can choose either to smooth out the shading with a tissue or not to – choosing not to can give your wood texture a rougher look. Ultimately, it doesn’t make much difference whether you do or don’t.
Draw jagged lines across the drawing. Don’t be too crazy when doing this or it may look too unrealistic. The easiest way to ensure that your wood texture looks realistic is to use a wood grain reference picture.
Continue to add the grain details. If you don’t have access to a reference picture, I can offer a few general tips. After drawing a couple of well defined lines (in the previous step), add finer lines which follow the contours of the stronger lines. Make sure you vary the length and strength of these lines. You can also add some growth rings as I have done, but make sure that you draw the lines around these rings.
Once you are happy with the look of the wood grain, shade over it very lightly until you get the tone that you are looking for.