1) go to the dd-wrt downloads page for linksys
http://www.dd-wrt.com/dd-wrtv3/dd-wrt/downloads.html
2) select your model of router from the list on the left
3) download the mini version for your router
4) install the mini version just like you would install a Linksys firmware upgrade.
5) If you want more, download and install a more involved DD-WRT firmware. the mini version will do what you want.
6) follow these intructions:
Wireless Bridge - DD-WRT Wiki