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.

if buddy's using Windows XP, just tell him to go to the control panel and find the Network Setup Wizard, and follow the instructions.  should also be in Windows 98 and Windows ME, but i don't have a 98 or ME system to confirm.

this is assuming that it is, in fact, a switch that the network's running through, and not a router/hub thingy.  otherwise, just follow the instruction manual for the router/hub thingy.