Here is a comprehensive guide from google.

1. From the main Outlook window, click “File” on the top of the window and click “Options” on the left side of the window. This will open the options window.
2. Click the “Mail” section on the left side of the window, and click the “Signatures…” button on the right side of the window. This will open the signatures window.
3. In the top-left quarter of the window, click “New” to add a new signature. Give your signature a name and click “OK”.
4. Your new signature will now be selected. Enter the text you would like as your signature in the text field in the bottom half of the window and click “OK” once you are done to save the signature.
5. In the top-right quarter of the window, select the signature you would like to use as your default from the respective drop-down menus.
6. Click “OK” to close the options window.

In addition of automatically adding a signature to your messages, you can also choose to manually add your new signature to a mail message. To do so, compose the message normally and then:

1. In the “Message” section of the mail's top ribbon, click “Signature”.
2. Choose the signature you would like to use. It will automatically be added wherever you selected in the message.