Jon, assuming you're referring to doing it through the network (as opposed to connecting them directly via Firewire cable), on one Mac you go into System Preferences, to the Sharing panel, and turn on File Sharing. Nothing else, just file sharing. Then, on the other Mac, you click on the Go menu and go to Network (or you click on Network in the sidebar on the left side of an open window). You should then see the first Mac listed there and can double click on it to connect.