Make a new layer. Ok now here's an important part. Try and get your Foreground brown color as close to mine. As you can see, its already in the default swatches. Apply the clouds.
Ok lets bring in the detail.
Unsharp Mask, Amount 500, Radius 1.3, Threshold 0. After that, I made a new layer and just loaded some of the brushes that came with Photoshop and made some blots with the Wet Edges option checked on the brush. Ok this is pretty much the effect. If you want a more darker rough look then continue to the next step. Turn down the brightness a little and then turn up the contrast and play with it until you get the look you need.