Letterheads are created for use with HTML email templates, but the letterhead must be available before creating your template, unless you edit a template at a later time.
From Setup search Letterheads from the Quick Find box > click Next or New Letterhead. Tick Available For Use, add your Label and Description (if necessary), and Save.
Next you'll edit the details of your letterhead to brand it to your company:
*Tip: If you're unfamiliar with your company's official branding colors in HEX codes, then please refer to your browser's extensions or applications, such as "ColorPick Eyedropper" in the Google Chrome extension that will scan the HEX code for you.
Please reach out to Product Support (firstname.lastname@example.org) if you have any questions about developing new letterheads.