restore a macbook pro snow leopard to factory specs

purchased a new Air; used Time Machine from my Pro to the Air, and now I need to reset the Pro to factory specs so I can sell it. The Pro is still on the latest version of Snow Leopard 10.6.8. How do I do this restore? What I've found on YouTube doesn't work; when I reboot from "off" holding down the Option key -- my only option is to click on the Mac HD. any help appreciated.