Leave one router connected to the modem exactly as it is. I'll call this router 1. Then, take the other router (router 2) and connect an ethernet cable from one of its' LAN ports in to one of the LAN ports on router 1. So now you should have something like this:
Internet => Modem => Router1 => Router2
Before you do this, you'll need to access the web management program on Router 2. See the manual for instructions on how to do this. You'll need to change the IP address to make sure it isn't the same as router 1 or anything else on your LAN. (If you don't know, you could always pick something obscure like 172.16.84.55). You will also need to disable the DHCP server on router 2, so you don't have two different DHCP servers running.