How do I connect a Linksys router to a Belkin router as an access point?

Okay, I have two routers. One is a Linksys WRT120N (I also have a WRT54G2 V1 if the current isn't compatible...) and I also have a Belkin F5D7230-4 router. The Linksys is connected to my cable modem and has good enough signal for all of my downstairs, but my upstairs has little to no connection in certain places (MY ROOM for example!!) so I'd like to configure the Belkin router to be used as an access point or repeater. Keep in mind that there is no wired connection in my upstairs so I need to configure it as a repeater that won't be hardwired to the original router. I'm willing to update to 3rd party firmware or anything if need be. I don't care if it's something that will potentially brick my router or void the warranty, if it will work...please help me do it! I've read everywhere that it's supposed to be this oh-so-easy task...but uhh...I'm not having very much luck!