First, choose an image. Choose something with an area with a lot of dirt, cracks, or texture. You may use this picture if you want to.

Second, desaturate the image (Ctrl+Shift+U or on Mac Apple +Shift + U)

Third change the Brightness/Contrast (Image/Adjustments/Brightness-Contrast).

I used: Brightness: +13, Contrast +60

Fourth, erase everything you dont want in the brush.

Fifth, change the Brightness/Contrast again.

I used: Brightness +1, Contrast +60.

Now Go to: Edit/Define Brush.

And your done!

Final brush looks like:

FOR GRUNGE ON TEXT

Type the words.

Now go: Layer/Rasterize/Type

Now use your open your erasers up and select ur grunge brush. Erase some of the words with it.

Link to the Guide to Creating Grunge Brushes!