Setting Developer Options

The first step is to enable the developer options on your Nexus. Go into your phone settings, and choose "Developer Options". If you don't see this option, go into "About Phone" and tap on "Build Number" until the option appears. Once you've entered the Developer Options, enable Android Debugging.

Connect your Nexus to the PC using a USB cable. To ensure connectivity

$ sudo lsusb

You should see a device in the list similar to as follows:

...
Bus 001 Device 055: ID 18d1:4ee1 Google Inc.
...

If you haven't already, you'll need to add the platform-tools folder in the Android development framework to your system path to get access to tools like adb and fastboot. Once you've done this, you can use adb to query your phone.

$ adb devices

The device should show up as follows:

List of devices attached:
<random string here>            device

You can then reboot the phone into the bootloader. You can also do this manually by turning the phone off, then holding power and volume down (volume up on newer Nexus models).

$ adb reboot bootloader

The next step is to Unlock the Bootloader.