Start by drawing something on the center of your page. I decided on a pattern of overlapping elephants. (And please forgive my terrible pictures from my phone.) Make sure your drawing doesn't touch the edge.
Next, cut the drawing in half vertically and horizontally. Make sure to do this as precisely as possible.
Line up the four corners and tape them together, again as precisely as possible, then fill up the center space with the remainder of your pattern. Again, make sure that your lines don't go out to the edges.
Next, I scanned the image and inked over it in Photoshop.
Then colored in Photoshop.
Next, select your entire image, and, in Photoshop, go to Edit>Define Pattern.
Now, create a new file, or select an area in an image that you want to fill with your pattern.
In Photoshop, go to Edit>Fill and from the popup window, choose Pattern. From the Custom Pattern dropdown, choose the pattern you had just defined from the list and click OK.
You may have to tweak your pattern a bit to make sure the lines and colors line up, so keep your original file with your pattern open so you can make changes to it, and then you can define your pattern again in Photoshop. You can easily delete custom patterns by right clicking on them and deleting them from the list in the Custom Patterns dropdown.
Here's the final pattern! Quick and easy!