Help loading DD-WRT onto router?

I just read an article online that said you can install DD-WRT firmware on a router to basically make it a wi-fi adapter for an xbox 360. I found an old Linksys WRT54Gv2 router and downloaded the right firmware for the job. Then it gets confusing. I am currently connected to my home wi-fi, so when I try to browse to 192168.1.1 online it sends me to that network connection. But it won't let me go to 192168.1.1 when I'm offline. How can I get to the Linksys upgrade screen that's supposed to appear when I go to that IP address?