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Define Pattern
First make a new image 20x20 pixels, then use the Zoom Tool to zoom in until 400%.
Use any tool to draw anything you like on the picture:
Go to Select>All or press Ctrl+A, then go to Edit>Define Pattern.
You can close the picture now, photoshop already define the pattern.
Create a new image, this time use 300 x 300 pixels.
Go to
Edit>Fill.......
                             
Use:  Pattern
                       Opacity:  100%
                           Mode:  Normal
You have a pattern now!