Signature
A signature from the leader of your organization is a great way to personalize your letters and make them feel more authentic.
Create your signature just like your would any other image file. You can use a script font to make one you like, or scan your actual signature as an image.
PRO TIP
Make sure the background of your signature is white or transparent. If you have other colors or pixels in the background, they will show up on your letter and make it look unprofessional because it won't match the letter.
This signature would print exactly like this and look like a grayish box on a white letter. Your graphic designer can clean up your signature file to remove the background color. No designer? There are many services online to do this inexpensively - Fivrr is an example (unaffiliated).