Click on 'File' in the upper left hand corner of Outlook
Click 'Options' in the lower left menu
Under 'Mail' at the top of the left hand menu, Choose "Signatures" mid-screen on the right side:
Click 'New', name your signature and then create it in the window at the bottom of the screen:
If you are doing this as the result of a new email account related to an acquisition, please add something along the lines of the following below your signature:
******* Please note my new e-mail address and add it to your contacts *******