Managing Salesforce Classic HTML Letterheads
Follow the instructions below for more information about using letterheads in HTML email templates in Salesforce Classic. If you need to create a new email template it is recommended to build the new template directly in Cirrus, Salesforce Lightning or Campaign Monitor depending on how you're planning to send your message.
Switch to Salesforce Classic
Click on your user icon in the top right corner of the screen, then select "Switch to Salesforce Classic".
Search for "Letterheads"
Create a New Letterhead
Edit Properties and Save
Make sure you check the "Available For Use" box