Please follow this guideline to install and activate the plugin. 

After that, follow this steps to get credentials of Square:

1. Go to Square Developer Dashboard.
2. Create/choose an application.
3. In the left pane, choose Credentials.
4. At the top of the page, choose Production mode for a production access token or Sandbox mode for a Sandbox access token.
5. Scroll down and get the access token.

6. Add this access token in payment method settings.

7. Then from the left pane, choose Locations.

8. Get Location ID from there and add in payment method settings.