The Tutorial

Download Image (right click)

1. Desaturate your image : Image > Adjustments > Desaturate (command/control + shift + u).
2. Then you play with the levels : Image > Adjustments > Levels (command/control + l).
3. Drag the outer sliders more towards the middle, so you get a more back and white effect.
4. Make sure you don't exaggerate, make sure there are some grey areas left.

Select a suitable area and clean up

1. Now select the Lasso tool from the Toolbox and select an area which you think would be perfect to create the brush.

A grunge brush is born

1. When you're done, go to Edit > Define Brush Preset and name your brush.
2. The brush will appear in the brush palette now (last one in the row).

Now Continue with the Part Two

(extra credit)
Incredibly Realistic Burnt Parchment in Photoshop