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Let's assume you have an internet connection and you want to share it. There are two main ways to do that.

   internet                           pc1
1. ----> |router| ---> |switch| --->-<
                                      pc2 ..etc

  internet
2. ------> |pc1 (router)| --> pc2..etc

I'll explain the second way (it is easier and requires one less machine).

1. Install a second network card to the first PC.
2. Connect the two PCs (using crossover cable).
3. Let's assume that the first card (with the internet) is called eth0 and the other one (for the sharing) is called eth1. (If those two keep switching at every boot read this ).
4. Configure the second network card with:
IP: 192.168.0.1
Netmask: 255.255.255.0
or enter in a console (as root)

ifconfig eth1 192.168.0.1 netmask 255.255.255.0
ifconfig eth1 up

5. Enter that same information in your /etc/rc.conf so that this card is set up correctly earch time your computer starts.

eth1="eth1 192.168.0.1 netmask 255.255.255.0"
INTERFACES=(lo eth0 eth1)

6. Enable packet forwarding. To do so, write a "1" to /proc/sys/net/ipv4/ip_forward with:

echo 1 > /proc/sys/net/ipv4/ip_forward

Then edit /etc/sysctl.conf and set 1 to the net.ipv4.ip_forward (It should be net.ipv4.ip_forward=1). This will make that change persistant after a reboot.
7. (If you haven't already) Install iptables and enter this rule (for the forwarding of the internet to the second PC) and save iptables.

pacman -S iptables
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
/etc/rc.d/iptables save
/etc/rc.d/iptables start

8. Add iptables in your DAEMONS array in your /etc/rc.conf so that it is started each time.
9. Go to the second PC and set:
IP: 192.168.0.2
Netmask: 255.255.255.0
Gateway: 192.168.0.1
DNS: The same DNS as the first PC
10. That's it! The second PC should now have internet.