When in SUmail, you can set up or change your Signature by clicking the Gear icon (Settings) located between the Bell and and the Question Mark on the top right corner of your screen.
In the search field under the Settings menu
You will see a section called "Email Signature." Enter the text for your signature in the box provided and use the toolbar to customize the signature by changing the font, text color, bolding the text, inserting a hyperlink, etc. You can't include an image in your signature in the Outlook Web App (SUmail web interface).
If you want your signature to appear on all outgoing items (including replies and forwards), check the box for "Automatically include my signature on messages I compose." Be sure to click "OK" in the bottom-right hand corner when you are done.
You can also include your signature on individual emails by going to the message toolbar in a new email message drop down menu and selecting "Insert Signature."