Well, assuming you're using Windows and your Ethernet cards are already configured & operating, and that you're talking about Direct Connect (no router or hub or DHCP-assigning thingy)... Then it's not too hard.
You'll need to configure each of the two machines with a Static IP, in your TCP/IP Settings (Control Panel->Network->TCP/IP Properties). Give one machine 192.168.5.1 and the other 192.168.5.2 . You'll probably have to reboot, and then you should be able to see the two machines in the Network NEighborhood. Just share a folder and away you go.
If you want more info, provide more on what you're doing