Step 1: Ready the canvas!
Set up your canvas by selecting the height and width. I chose 300x300 because it has alot of space to work with and its a square so it'll be easier to tile.
Then select the fill bucket tool and choose a shade of brown for the base.
Then, fill the 300x300 canvas with the brown color using the fill bucket.
Step 2: Make some noise!
Then, mess with the settings to get a nice bummpy(-ish) look.
Soon after you need to create some motion blur to give it that nice wood grain look. ( Filters -> Motion blur...)
Step 3: Look sharp!
The seamless function blurs some areas of the texture so you'll need to sharpen the image. ( Filters -> Enhance -> Sharpen...)
Step 4: Make it bumpy
Duplicate the layer: Layer -> Duplicate layer.
Desaturate the duplicate: Layer -> Colors -> Desaturate
Bump map: First, select the original layer. Dialogs -> layers, click on the colored layer. Then, use the desaturated layer as a bump map Filters -> map -> bump map...
Now, all you need to do is delete the desaturated layer. Using the layer dialog select the B&W layer and delet it by pressing the trash can button.
Step 5: Stain the wood
First, select the paint brush and a black color.
Set the mode of the brush to 'overlay' and paint where you want to stain the wood texture.
Wala! You have a beautiful seamles wood texture!