How to set 2nd LinkSys Router as AP?

You have to wire (one LAN port on the first router, to a LAN port on the 2nd router) two routers together to do that. If this is what you want.... you can set it up like so:

Change the IP address of the 2nd router to 192.168.1.2 (assuming your first one is 192.168.1.1)
Turn OFF the DHCP server on the 2nd router

This is optional:
the SSID and encryption keys to the same on both routers.

Hope this helps