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.

Most consumer router/hub combos I've seen have a separate ethernet port for the ADSL modem only. So quite simply, the modem goes in that port and gets the internal address of 192.168.1.1, the router takes 192.168.1.2, and then the router's DHCP hands out addresses, the subnet mask and gateway address to the LAN clients. Pretty easy.