We will start off with this royalty-free image we downloaded:
Now, our aim is to create a black and white image, with the correct contrast to make a successful brush. First, goto Image > Adjustments > Desaturate. This will remove all color from the image:
Next, goto Image > Adjustments > Levels, and adjust the slider to get rid of the colored background. We can still leave all the small details like the small cracks.
Next, press Ctrl+A or goto Select > All, then goto Edit > Define Brush Preset. This will bring up a box for us to name the brush:
Give it a name and PhotoShop will add the brush to the currently selected brush set. So if you choose the brush tool, and then goto the different brushes by clicking on the drop-down menu at the top, you should see the brush we just added at the bottom of the list.
We will have to make sure we save this brush set if we want to keep our custom brush, because if we do not, our custom brush will be lost when we change brush sets.
To save the brush set, we click on the small arrow (
) at the top-right of the brushes drop-down, and then choose Save Brushes. We will then be prompted for a filename. Once saved, you can load the brush set in the same way as you would any other - from this menu.
Now to use our brush we simply select it from the list and paint on our canvas !
More Tutorials: 
