First of all, we create a network
/network add -nick
-user /network add -nick fred -user fred freenode
Next up, we add a server to the network - this step is so we can add a number of alternative servers for the same network, I never do though.
/server add -noauto -network
/server add -noauto -network freenode irc.freenode.net
You can also use -auto rather than -noauto above to reconnect to this network every time you use irssi.
Finally, we connect.