Port forwarding through a DSL modem and two Airport Extremes

Hi, I'm new to these forums and this is my first post, so I apologize if this topic has already been answered elsewhere. I searched the forums and google for the answer but I haven't had any luck.

Here's my situation: I'm trying to setup port forwarding so that I can remotely access Plex Media Server from my iPhone away from my home.

At home I have a Motorola 2210-02 AT&T DSL modem connected to the internet, connected to that I have an Airport Extreme (the latest) and connected to that, via ethernet, I have another Airport Extreme (same model). I have have what Apple calls a "roaming network" and it works flawlessly. Now of course I have to go and make it more complicated.

Can anyone advise me how to setup port forwarding so that I can access my computer through all of these devices?

A little extra info: I have checked portforward.com but their explanation of forwarding through multiple routers is over my head.

$20 bucks to whoever helps me get it working!

-Jack