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!