This tutorial supposes that you know how to move around photoshop.
1 - open a new document of any size and with the background set to white, choose yer brushes and create your background.
http://img261.echo.cx/img261/2417/tuty15bq.jpg
2 - select the gradient tool and open the gradient editing menu, where it says "solid" click on it and select the often forgotten "Noise" gradients (choose whichever one you want), now on roughness set it to 25%.
http://img105.echo.cx/img105/8273/tuty25rl.jpg
3 - create a new layer and use the gradient at whatever angle you want and set the blending mode to soft light and you're finished.
http://img284.echo.cx/img284/6014/tuty37xv.jpg