It's recommended that letters be written on official letterhead, and include a hand-written or digital signature of the writer.
How to add letterhead:
The easiest way to include letterhead is to have the writer type the letter in a template already formatted with the letterhead.
Many universities and departments have standardized letterhead templates they provide to professors and administrators. Most medical practices will also have letterhead templates for doctors and staff.
How to include a signature:
Usually, the easiest way to include a hand-written signature is to Print, Sign, Scan, and Upload the letter. We recommend this process for professors or doctors who are less familiar with computer technology.
A quicker option is to create and include a Digital Signature on Interfolio. Yoir letter writer can see this link for detailed instructions on creating a digital signature.