Example: here

1. Find a pattern. I took the first I found which you can find here and it's made by Nohemi-Muniz at deviantart.com , if you use it, credit!

2. Make the pattern the background on your page like I have done on the picture below.

3. Yeah, I guess it looks kinda noobis lol. And I don't think it's the professional way they do it. But hey, it works !
First take a screenshot of the page, just like I have done above. Paste it as a new document in Photoshop and zoom in to the top left corner using the zooming tool. Make sure it stands on " + " so you go closer. Also make sure you've marked "Resize Window To Fit". Like I have done on the image below:

Now lets continue with copying the pattern exactly as it is on the page. Choose the Rectangular marquee tool ( Click M for auto choose ) and make sure you copy exact within the grey borders of your internet browser and just drag it down. Like I have started on the image below:

K.. Now you're at the bottom:

You have now marked the whole page, copy it, and paste it in a new document and you can now do whatever you want on the pattern and make sure it fits !

4. You have now like I have below.

K. This is now your own resolution's size. And everyone don't have that screen resolution. Therefore I always crop a part of the top left corner. Like this:

Screenie 2:

If I now make a very quick layout just to show you.
I made this:

If you then go to this page you'll see how it looked.

This is what you've just done.