MAC St
Go to Outlook | Preferences
Click on Signatures
Click on the plus/minus to add or delete a signature
In Word you can markup a signature and copy it your new signature or you can open a existing message and use someone-else's. Once you are satisfied, click Save icon. (Don't forget to set when to use the signature e.g. New messages)
Windows Steps
At the top-left click on File
Then at the bottom click Options
ON the left- click Mail then on the right Signatures
Click on New
Once you are satisfied, click Save. (Don't forget to select when to use the signature under New messages)