It is not very hard! First from “System” -> “Setup” -> “Network” assign the IP address you want to the LAN and WIFI interfaces. If you want you may also bridge them to be in the same collision domain. In the same screen you setup the PPPoE to terminate your internet connection. To enable nat you need to go to “Network” -> “Router” -> “NAT” and move your newly created ppp0 to the NAT enabled interfaces.
These steps give you a basic configuration to work with. Any other thing you want to do is documented here or in the documentation page.