Log in to Mailchimp.
Go to Account > Extras > API Keys.
Click Create A Key and copy it.
In WordPress, go to Plugins > Add New.
Search for Mailchimp for WordPress (MC4WP), install, and activate it.
Go to MC4WP > Mailchimp, paste the API key, and save.
Go to MC4WP > Forms, create a form, and copy the shortcode.
Paste the shortcode into a page, post, widget, or popup.
Enter a test email and check your Mailchimp Audience > Contacts.