There are a couple of great how-to guides for configuring Office Extend APs on the web. Once upon a time, documentation for the OfficeExtend feature was a rarity. Back then the only guidance was from this great post from Blake: Cisco OfficeExtend AP600. Now we have the config guide from Cisco: Aironet 600 Series OfficeExtend Access Point Configuration Guide
I used Blake's post about OfficeExtend