There are two ways of doing this
so I suggest you to get grunge brush rather than making it manually,
for download brush check this :
and if you have time than:
make a document erase the corner's according to your need and you'll get the same output..
I did the same to show you,its kind of heavy/ugly borders, coz i did not reduced the eraser brush size, i realized after completing it :(:(
Hope you got the idea, That how to do this
There is always the wooden table method:
take a brush, paper, and some black ink and then sketch a rectangle.
Photograph it with your cell phone or high-res camera, convert to B&W.
Use this image as a
Using a layer mask has the advantage of being non-destructive.