Connect directly to your primary router. Pick up an IP from it and note the gateway (probably something like 192.168.x.1).
Connect directly to your secondary router and pick up an IP. Log into it and disable the DHCP on it. After that is done change its IP address to 192.168.x.2. It will probably reboot and you will loose connection to it.
After that run a cable from a LAN port on the primary router to a LAN port on the secondary router.
You should have a full connection now. Your secondary router will act as a switch/access point.