This guide is upgraded due to process being much much simpler than it used to be to install DD-WRT on Asus N66U.
1. Download the files here. (taken from wiki)
The new K3.x Builds seem to take all the complexity out of the equation on what build you have and the CFE issues. The reason I had to reinstall DDWRT was due to Asus needing to be constantly turned on and off from internet connection issues. I thought it might be Asus issue so I went through process of returning to stock firmware (which isn't fun and took hours of experimenting). From stock firmware it was super simple upgrade to select right K3.x build that picture above explains. The actual problem was due to modem SB6141. I read on some forums that Comcast was upgrading firmware and causing internet connection issues so I thought Cox might had done same to me. Sold the modem and got a Zoom, first one I have owned, and problems went away.
2. Do the 30/30/30 reset - Note for this router don't use reset button, instead replace reset with WPS button
3. Go to http://192.168.1.1 and login to router that now has DDWRT installed!