How to create/change signatures
Write your signiature using your text editor (notepad, wordpad...) and save onto your computer somewhere convenient.
Run NN
Edit
Preferences
Mail & Newsgroups
Identity
Choose
Locate the notepad document you created, and click open.
Click "OK" at the bottom of the NN Preferences window.
The next time you create a new message your signature file will be displayed in the message by default.