You connect a computer to the second WRT. Nothing else. Turn on the WRT, boot the computer. Go to http://192.168.1.1/ . Configure the LAN IP address to 192.168.1.2 (the address must be in the LAN of your first WRT and must not be inside the DHCP server range). Turn off the DHCP server. Save the setting.
Unplug the computer. Now connect the cable from the first router into a LAN port of the second WRT. Do not use the internet/WAN port!
I guess you also want a roaming network, i.e. you want to be able to move your wireless clients between both routers without loosing connection. For this, both routers must have identical SSID and wireless security settings. The channels should be different and non-overlapping. Select channels 1, 6 or 11.