First, you will need to go to Settings > Templates - Emails/Texts/Forms. Click "Create a Template," "Duplicate a Template," or edit an existing template, depending on what you need. After the email template editor opens, you can edit the template how you would a normal word document. You can add dynamic content, such as custom fields, by clicking "Insert Datafield" and then finding the type of data you need.
Once your template is ready, head back to Settings > Automatic Actions, pull up your Send email notification action, and navigate to the Actions page. Change "Email type option" to "Use Existing Email Template," and under "Select email template," find the template you just edited.