Making a Grunge Brush
1. Open a new image size 200 x 200 with a transparent background.
2. Using black, scribble all over the canvas with a variety of different default Photoshop brushes.
3. Click on the eraser tool and messily erase parts of your picture that have too much black. You want some black, but you also want the textures from all the different paint brushes.
4. Go to Edit- Define Brush, name it and you're done!
To use your grunge brush, click on the paint brushes and scroll to the bottom.
Here is a smaller example:
