Go to 'My Home' listed in the topmost orange bar of the 'page'....
Go down until you get to 'Main Configuration'....
Top line bears this legend:
'Personal information, email, password, etc.'
Click on 'edit' in Right column....
Scroll down, until you get to a box marked 'signature'...
Enter your text, read, and proof-read, and when satisfied, go right to the bottom of the page and click 'submit.'