Download Unity Hub from here ​https://unity3d.com/get-unity/download

After installing Unity Hub, open it 
Press "Installs" tab
Press "Add"

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 "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" 

Remove Vulkan 

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.