1. First of all, as usual, create a new document to make the brushes in. I've used a document size of 400 x 300 pixels with a white background.
2. Now with one of those large, soft brushes that come with Photoshop, brush a few times on the Background layer with black.
Settings:
Brushing:
3. Now you need to find a grungy/dirty type of texture on the internet. I've used next dirt texture photo I took a fair few months ago.
Open up your photo and save it as a PSD file onto your desktop or somewhere.
Now back to your brushing, be sure the background layer and brushing layer are one (Background) then apply Filter > Distort > Displace, use settings similar to these and choose the PSD file afterwards.
You should be left with a result similar to this:
Looks alright, doesn't it?
4. To finish this off, get another grunge brush either you made or someone else, and brush a little bit over the edges with white.
You also might want to apply Sharpen and fade it to about 25%.
Here's an image I made with my grunge brushes:
