Before enabling SPF and DKIM you will have to validate your domain in Mailchimp.
Configure DKIM
Setting up DKIM in Mailchimp is super easy, and only requires adding two CNAME records.
For the most up to date information please refer to MailChimp's guide on how to authenticate your email.
Configure SPF
Adding SPF for MailChimp for DMARC purposes is not needed because the Return-Path address of the emails is set to Mailchimp's default domain in order to track bounces on your behalf. Therefore, SPF will not pass for MailChimp from a DMARC's perspective, however, as long as DKIM passes, the emails will be DMARC compliant.
If you need further clarification on what SPF and DKIM are please refer to these articles:
Instantly test your Mailchimp configuration
OnDMARC’s free tool Investigate lets you verify your Mailchimp configuration set up to ensure they actually authenticate your emails correctly.