Start up irssi, then:
   /connect irc.freenode.net
   /nick MyIRCNick
   /SERVER ADD -auto -network freenode irc.freenode.net 6667 <password>

      (you may have to shutdown and restart irssi at this point for it to
         recognize the network name "freenode" in the next step)
         /CHANNEL ADD -auto #crux freenode