I like to connect my Airport Extreme Base to my ISP Cisco. The Wifi signal of the Cisco is very weak so the Airport has to do all the router functions such as DHCP and WIFI What to do on the Cisco before I can install de Airport. Yes I have to set the Cisco in Bridge mode... but then what to do next? Have I to NAT on the Cisco?
I think it depends on what you want to achieve. You may still want the Cisco to act as the gateway because it has more robust features than an Airport.
If you only want to replace the Cisco's WiFi signal, disable the Cisco's radio. Then connect the Airport and disable DHCP. Probably a good idea to assign it a static IP as well.
If you want the Airport to handle everything, like DHCP and firewall, then yes - bridge mode.