I have CenturyLink DSL service in an ex-Qwest service area. After several years of very poor security practice, I have been able to switch to a new firewall.
In general the steps are:
get your PPPoE credentials from CenturyLink if you haven't already put the CenturyLink gateway into transparent bridge mode and allow it to apply/reboot make the WAN interface on the FortiGate use PPPoE, enter your creds
Use the virtual IP setting to do port forwarding in on your IPs.
On your other IPs you probably don't need all services forwarded in so you'll need to do a firtual IP for each service you make