These are the steps you'll need to do in order to setup a signature file in Outlook 365.
1. First, you'll need to open Outlook and then from the main screen, click on New Email in the upper left corner.
2. From the window that opens, click Signature and then Signatures (as seen below).
3. When the Signatures and Stationery window opens, you can create a new signature file. Click New.
4. Type in a name for the new signature.
5. Your named signature will now show up at the top of the box, and you can create the actual signature block at the bottom of the same box. Once you type in your information for the signature, make sure to click Save.
6. Choose your default signature by selecting it from the drop down menu.
7. Close all windows. Now go back to the main Outlook page, and click New Email. Your newly created signature should populate in the new email window that opens.
***Note: Any signature changes in Outlook 365 will also be reflected in the Outlook Webmail Application (OWA).