It's like setting up Internet Connection Sharing in Windows.
Step 1: Go to the address of the gateway 192.168.1.1 in browser
Step 2: From home-network-help.com:
Setting WAN (Wide Area Network) Setting
Static IP - Choose this option to set static IP information provided
to you by your ISP. You will required to type in all network
information manually if select this option. This option is mostly used
by business users. Lot of works if use this option, try to avoid this
PPPoE - Choose this option if your ISP uses PPPoE. Your ISP will
provide you with a username and password. This option is typically
used for most DSL services.
PPTP - Same as PPPoE, This option is mostly used by DSL service users
with provided username, password and IP information.
L2TP - This option is mostly used by DSL service users is Europe with
provided username, password and L2TP dedicated server information.
Telstra Cable: This option is mostly used by DSL service users with
provided username, password and Heart Beat dedicated server
As a cable modem user, I will choose Automatic Configuration – DHCP
option. I won't clone MAC address because the service registration is
tied to cable modem's MAC address.
At the same setup page, set up the IP address for the router. This
will be the router/gateway IP address that you will set on your
network computers. The LAN IP address is private in your home network
and cannot be seen from the internet. I set my router with IP
192.168.1.1 and subnet mask 255.255.255.0.