Back Button Autofocus?

On a Nikon D40? Does anyone know how to set this up?

Thanks!

Added (1). Asking someone who has done this on this camera and knows how. Not wanting a simple "check the manual" sort of answer, please!

Thank you!

Added (2). Thank you George! Perfect! I don't know how i missed that, i was on that page earlier today haha

Try your owner's manual. If you don't have one go to the Nikon site and download a pdf version.

After some searching, I learned something new today!

Read this and scroll down to #12. There's how you can custom setup your AE-L/AF-L button for back button autofocus. Incidently, I'd suggest downloading and printing off all of Ken Rockwell's guide here. I've done the same for every Nikon DSLR that I have and keep them in folders for reference.
http://www.kenrockwell.com/...custom.htm