Part 1 : Go to the git repository and download/clone it https://github.com/googleads/googleads-mobile-unity
Part 2: Download unity google admob latest plugin https://github.com/googleads/googleads-mobile-unity/releases
Part 3 Create a Unity Project and import the plugin
Part 4: Go to following directory of the repository you download of step 1 : google admob plugins\googleads-mobile-unity-master\googleads-mobile-unity-master\samples\HelloWorld\Assets\Scripts
Part 5 : Copy the script to the assset folder
Part 6 : Attach the script to MainCamera.
Part 7: It should show default ads when you will build it for android or iOS .
You can now customize it for different uses like game over case and etc.