Ask Swantide to install Mailchimp on your SFDC.

When that is complete, do step 1 - 3 of setup on this page

https://mailchimp.com/help/connect-mailchimp-for-salesforce/

A step by step guide for those steps can be found here.

Once you finish step 3, ping Swantide and we will finish permissioning, sync settings, and activation of layouts!