Note the build is currently in let's say beta stage considering the Android 4.4 is not even out yet.
How to install? Just like other ROMs, flash in custom recovery. I heard the TWRP might cause problematic in some ways, therefore I recommend you the ClockWorkMod recovery.
Be sure to wipe data otherwise your device won't boot up.
Update: Now, as the development continues, you can actually get Android 4.4 on your Nexus 7 as well!
Installation is the same - flash via CWM, then GApps, don't forget to wipe.
New update! The Android 4.4 ROM has also been ported (again) for Galaxy Nexus! Installation goes as usual - be sure you're downloading the right files, tho! Note this is an early release that contains some graphic glitches but it's still usable as a daily driver and has a very smooth performance. Worths a shot!
Again, next update! This time, and it's very surprising, Android 4.4 support has been added to old device Nexus S! Google has cut the support and stopped developing for this device in Android 4.1.2. Now it's the time to update it again! Supposedly, it's running pretty fast.