Cable modems typically come with an Ethernet cable. Plug one end of this cable into the matching jack in the back of the modem. On a router, there are typically one to five similar jacks. If there is more than one, one will be either separated from the others or colored differently. This is your WAN port. Plug the other end of the Ethernet cable into the WAN port. Plug in the cable modem's power adapter, and then the router's.