Before we start the setup, we will assume you already have a google merchant account, if not I invite to follow this : How to create a Merchant Center Account or directly visit merchants.google.com
Link your website to your Merchant Center Account
Verify and claim your website URL :
In the Business Information Section
Then you will to verify your domain by adding a meta tag to your layout ( head block section).
Once this done you just need to ask google merchant center to verify your domain.
Get your Google Merchant key for authentication
Merchant Center can automatically create a Google API Console project and associated service account key for you. Your application will authenticate with this key when using the Content API for Shopping.
Select Content API from the three-dot menu in Merchant Center
Select the Authentication tab.
Click the + button to create a new API key.
A file called content-api-key.json will be downloaded
Now open content-api-key.json in a text editor and copy its content to your app store settings as well as your merchant ID.
The setup is now finish you will just have now to configure your Tax & Shipping settings. ✊
Next Step: Setup your feeds in the Admin Area.