They won't explode, no worries. The reason Birne said that is so that the second router that will work as an access point won't be held back by something slower. Wireless G is 54Mbps and Wireless N is 300Mbps (theoretical).
The Wireless N router will be functioning as a router. Then you just have to make the Wireless G router an access point.
Disable DHCP, give it a different IP address and change SSID and password if desirable. Connect a wire between the LAN ports on your main (Wireless N) to your Wireless G's LAN.
Do not plug it into Wireless G's WAN (unless you're using DD-WRT on that, and have assigned the WAN port to the switch).