Download the Google Cardboard SDK from HERE
Open a new project in Unity
Import the downloaded unity package into Unity project
Select "All" and Click "Import"
Go to Build Setting
Click "Android" tab and click "Switch Platform"
It will take few seconds to switch to Android platform.
Click "Player Setting"
In the "XR Setting", Click the tick box of "Virtual Reality Supported"
It will take few seconds to import virtual reality setting
Click the selection box in Virtual Reality SDKs and click "Cardboard"
"Cardboard" SDK is ready now.
Go to "Other Setting", select "Vulkan" and click the - button to remove "Vulkan"
Your all setup is done for Google Cardboard SDK and start building your scene.