... Install [link|http://www.fwbuilder.org|FireWall Builder] It rockssss the house. A bit confusing to People unfamiliar to the ways of Traffic Rules and such...
But, It is a Killer! Lotsa Docs and a Wizard... Just RTFM and you should be okay. If not, I can "help you" with a version of a script I have...
I manage about 12 firewalls setups with it. A few NAT, a few MASQ, a few Mail and HTTP only, some as a traffic checker making sure it falls into an "allowed" category, if not it's dropped.