From the main admin CMS on the left hand menu tab, navigate to 'Stores' then click on the hyperlink that is the storefront name in which to open up storefront settings.
(Please Note: Before you begin, you must enable Mailchimp for the store by clicking in the same store settings in ' Edit Store' a check box at the bottom of the page that will enable Mailchimp.)
Next click on the navigation bar marked as 'Mailchimp Settings' to open up the configuration table, below.
Begin by filling out, your already created mailchimp account settings, with the mailchimp provided Api and List ID key. For more information on how to generate an API key and find your audience( List) ID that please click on this below link.
Next enter in your storename that is associated with the mailchimp configuration and the email address that is connected to the storefront.
Finish by adding the mailchimp URL. To do this click on the button below and follow the processes which involve entering in your URL name and selecting from the drop down target audience and then clicking 'Get Code' in the example below
Finally, click 'Save' to configure and establish the integration to your storefront.