Achieving root access

The next step is to download the SU package, or a package containing SU and Busybox (which contains all your favorite linux commands and allows you to use them in a shell on your phone). The best way to find this is by searching google, since it is usually hosted by an ever changing file cloud download service of some kind. It should be in a .zip file: once you've downloaded it, you'll need to use adb to push it to the sdcard on your Nexus.

$ adb push /path/to/SU.zip /sdcard/SU.zip

You should see something similar to

XXXX KB/s (XXXXXXXX bytes in X.XXX secs)

Now that the package is on the device, you can install it using your newly flashed recovery image. From the menu on your phone, use the volume up/down keys to select "Recovery mode", and use the power button to select it. You should notice that the initial Google screen should now have a symbol of an unlocked padlock in the bottom middle.
Once in the recovery menu, select "install zip from sdcard", then "choose zip from sdcard". If you copied the SU zipfile correctly, you should see it in the root of your sdcard and should be able to select it. If you don't, you'll need to go back to the previous step. There is an option from the main menu "mounts and storage", if you still can't seem to find the SU zip on your sdcard, you can select "mount sdcard" and trying the push process again. There seems to be some sort of abstraction with newer Nexus models that makes this more difficult than it should be.

Now that the root binaries have been installed, you'll want to clear cache and restore factory settings.