1. For our example I'm going to create a rounded edge box with text inside. This tutorial is design for a white background (but don't worry - I will explain later how to do it without a white background). On a new layer begin by using the rounded rectangle tool with the 'fill' option selected with any color you like. If you use the 'paths' option right click it like shown and go to make selection.
2. Once your shape is filled with a color, make sure it is still selected (so it has the marquee selection like shown) and go to select>modify>contract. And use a value of around 6 pixels.
3. Once the selection is contracted, hit the 'delete' key on your keyboard to delete the contracted selection.
4. Now you can add some text.
5. Next find a picture of any city or an industrial based image. I used this: city-1.jpg
Add the image to your document on the top most layer. Your layers palette should look like this.
6. On your city image layer go to image>adjustment>threshold and give it a value of around 175-200.
7. Now go to select>color range and select any black area like shown. Use a value of around 200 for fuzziness.
8. Click OK. Now you should have many many small and large pieces of selection. Hit 'delete' on your keyboard. This will delete all black that was previously selected. Here is what you should see.
9. Now simply deselect everything (Ctrl+D) and there ya go! You can now move the grunge layer around to find the perfect look. This works because the grunge layer is white which is blending in with the white background.
10. If you have other stuff around your grunge affected area (which you probably do) you can simply make the grunge layer a clipping layer. Do to this, make sure your grunge layer is selected and located directly above your text or whatever object you want to have a grunge, and hit Ctrl+Alt+G on your keyboard.
That's it! I hope you enjoyed this tutorial.