Step 1: Install the Android SDK (really you only need the fastboot.exe and adb.exe)
Step 2: Install the Google USB Drivers
Step 3: Enable USB Debugging on the Nexus 7 (Go to "About Tablet" in settings and rapidly click build number 7 times to unlock developer settings, then go into developer settings and turn on usb debugging
Step 4: Open a command line from your platform-tools folder (wherever you have your fastboot and adb files) [easiest way in windows is to navigate to that folder, right click, and select open command line here]
Step 5: type the command "adb reboot bootloader"
Step 6: Once Nexus 7 boots into bootloader type the command in the same terminal window "fastboot oem unlock"
Step 7: On nexus 7 use volume up to highlight Yes then hit the power button to select it
Step 8: Once you confirm lock state says Unlocked you will want to scroll to "Start" using volume buttons and select it with power button
Be sure that you allow the device to fully boot into the OS, this will ensure that the data wipe is complete. If you omit this you will have issues with recovery and root later on.