|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.......
|You have a pattern now!|