Medium
Your medium will always be "email" and will be appended to your URLs automatically by Salesforce Marketing Cloud.
Source
Your source will be your email sending tool, and will be appended automatically by Salesforce Marketing Cloud.
Tagging by email tool
If you are using any tool outside Salesforce Marketing Cloud, you will need to append the UTM string (example shown below in bold) to all links in the email. Each URL in your email will need to be updated by hand to include this tracking.
undergrad.osu.edu/majors-and-academics/?utm_campaign=studl_student-event_fy24_fall-visit-october&utm_source=mailchimp&utm_medium=email
This is because these tools (Mailchimp, Constant Contact, Ticketmaster, etc.) do not append tracking to URLs, and therefore they will not come through to Google Analytics and will appear to be direct traffic. This does not allow us to accurately attribute traffic coming from email. If you need to add your links manually and would like the assistance of the url builder rather than writing them by hand, you can use the builder here (select email as your channel).