Open up a new file. Size 300×300 in pixels. Click the paint brush or click B and use one of the default round brushes. Set the size as 15. Don’t worry about the other settings.
Now take your brush and scribble random lines everywhere but leave some white space too. [screen shot]
Now go to Effects>>Distortion Effects>>Pixelate. [screen shot]
Set the block width and height to 20 pixels. And make sure to have Symmetric checked. [screen shot]
Now you can take the background eraser and erase away the background part that is black or you can leave it there.