Insecure Router Setup
A. Setup network interface (the internal one)
ifconfig eth1 192.168.0.1 netmask 255.255.255.0 bcast 192.168.0.255
if u have another network card, you could give it an address of 192.168.1.1 and have another network. Also, you could attach the uplink of the linksys to the router with a cross-over cable, turn off the dhcp server in the router, give the router an ip of 192.168.0.2. you can then use the router as a switch. (i think that's the setup, linksys tech support will tell u how to set it up).
B. Turn on forwarding
#echo 1 > /proc/sys/net/ipv4/ip_forwarding
there is a config file somewhere where you can set the value and not have to deal with it anymore. If you do it command line style the way i just showed u, you'll have to do it after every reboot.
C. Share the Internet connection
#iptables -A POSTROUTING -t nat -s 192.168.0.1/24 -j MASQUERADE
if you have a static ip to the internet:
#iptables -A POSTROUTING -t nat -s 192.168.0.1/24 -j SNAT -to <ip address here>
only choose 1 of those
D. Setup host
Give the computer connected to the router (or linksys switch if u set it up that way) an ip of 192.168.0.X (X = any number from 3 - 254)
E. Plug it all together (for computer-to-computer connections make sure you use cross-over cables as well as for the uplink-to-computer if you use the linksys router as a switch).
this setup will allow you to share your connection through a linux computer. it is insecure. you should check a tutorial on iptables for added security (DO THAT NOW).