If buy extender you are referring to extending your wireless network then the simplest version is
1. Take the brightbox you want to make into the extender, and connect it via Ethernet cable to a single computer and nothing else
2. Log into web interface and change LAN IP range to be a different one e.g. make router IP 172.17.17.1 and turn off the DHCP server.
3. Make sure the wireless security is configured how you want.
4. Disconnect the PC and connect the two routers via an Ethernet cable e.g. LAN port 1 to LAN port 1
5. The length of the Ethernet cable will then dictate where you can place Brightbox for best wireless coverage
Same sort of wording but for different hardware that I have previously written http://www.coolwebhome.co.uk/wap/configuring-wap.html