Setting up modem to share net through switch

A simple one for you guys:

How does one go about sharing an internet connection between 2 PCs on a modem and separate switch? I don't know the details of either because I'm asking for a friend, but it's a D-Link ASDL modem and a Belkin switch.

I assume the modem's ethernet cable goes into one port and the two computers in another two... but where does it go from here? I'm guessing they have to access the modem's menu and enable NAT of some kind.

Could anyone give a quick step-by-step runthrough that I can talk them through over the phone?

Cheers.

>>8
Second network card? My advice is just to buy a router. It costs same money, it is easier to install AND gateway comp doesn't have to run all the time.