Download Unity Hub from here https://unity3d.com/get-unity/download
After installing Unity Hub, open it
Press "Installs" tab
Chose Unity 2019.4 to install
In the next step, choose Android Build Support and Press "Done" for further default installation
If you are going to make your app for iOS, then choose iOS Build support
Press "Done " and follow the next steps for installation
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.
After Installation, Open a New Project
Goto "File" in Unity menu and click "Build Setting"
Click on "Android " and Press "Switch Platform" button
Goto "Window" tab and Open Package Manager
In Search Box, write "AR Foundation"
Selection 3.0.1 and click install
In Search Box, write "ARCore XR Plugin"
Selection 3.1.4 and click install
Goto "Edit" tab in unity menu and click "Project Setting"
In "Project setting" dialog box click "AR Build setting" and Press "Create"
In the new dialog box, click "Save"
Your installation process is done here.
Few other things to do
In the player setting, in "other setting"; choose Minimum "API level 24"
Here you Go. All is done.
You can now Add AR Session
Adding AR plane
Here all is ready. you can start working with your AR app.