Create a new document 3×3 pixels, with a transparent background. (The size of the image all depends on how big you want the pattern to look).
Zoom in 1600% then select the “pencil” with a 1 pixel brush size and the color black draw this shape. (see image below)
Now goto “edit > define pattern”, enter a name in the box that shows up and press ok.
Now open up an image you want to apply the pattern to, like a website header etc… im just going to use a blank canvas. Select the paint bucket tool, then at the top in the paint bucket menu where it says “foreground” change this to pattern.
Now click the little arrow and find your pattern.
Select your pattern then fill a layer with your pattern as you would if it was a color.
Thats it all done. The possabilities are endless.