Go to the Linksys E1200 support site and download the latest E1200 v1 stock firmware which supports bridge mode. There's not much reason to use DD-WRT or tomato if you're going to put it in to bridge mode. Make sure you reset E1200 to factory defaults after you flash the firmware.
And then follow the bridge mode instructions in link below.
If you want to tinker with DD-WRT or tomato on the E1200, then you can cascade it LAN to WAN in a double NAT, but make sure you change the E1200 IP address to 192.168.2.1.
Bridge mode is preferred over the LAN to LAN cascade.