How can I install a debug build of Android 5.0 AOSP on my Nexus Device? Where do I get the debug build? I tried Googling but still can't find step-by-step instructions to download and install debug build.
You would need to pull down AOSP in source form and build it for the desired Nexus target (assuming it is supported). Start here:
http://source.android.com/source/requirements.html
Once you are setup, look into the details of building for a specific device, found here:
http://source.android.com/source/running.html
Note that if you load your device with a custom image you'll be voiding any warranty, etc. Also, you probably don't want a full debug build, it will be very large and most likely not loadable on the device. You'll want an "eng" build, which has a reasonable amount of debug capability in place, even for system processes and frameworks.
Good luck!