Please note - if you're using any version of Windows lower than Windows 98 SE, look carefully at the WiFi adaptor requirements - many adaptors require Windows 98 SE or better.
Installing the WiFi adaptors
Install your WiFi adaptors - the instructions will accompany each card.
Typically, for USB adaptors, plug in the adaptor to a free USB port on the computer, then install the drivers when prompted. For PCMCIA cards, insert the card into a free slot in the laptop, and install the drivers when prompted. For PCI cards, power off the computer, open the case, inset the card into a free PCI slot, close the case, and power on the machine. Install the drivers when prompted.
Configuring the WiFi adaptors to "see" each other
After installation is complete, go into the network card properties and set the SSID to something unique. Make sure to set the SSID to the same value on each machine. If you don't, the network will not work properly.
Go into the network card properties and set a channel for the wireless network to use. Make sure you set the channel to the same number on each machine. If you don't have the same channel set, the network will not work properly.
Set the network cards to run in "ad-hoc" mode, instead of infrastructure.
Setting up Windows Internet Connection Sharing on the computer that the modem is connected to (the ICS Server)
Windows Internet Connection Sharing is really easy to set up on Windows XP - there is a wizard available at Start > Programs > Accessories > Communications > Network Setup Wizard.
Go to the host machine and start this wizard, following each step. If you don't have Windows XP, you'll have to do it manually. See this guide for detailed instructions for each version of Windows - http://www.practicallynetworked.com/sharing/