Sign into office 365
Select the gear in the u8pper right hand corner
Go to the bottom of the page and select
View all outlook settings:
Here you will chose compose and replay. That window opens up
and looks like this below.
Here you will type in your email signature in the box. Then
check off the automatically include my signature on new messages that I
compose, and you can also select the box below as well depending what you
want to do. Then you hit save.
Now you have your signature on all your emails J
The same holds true as to change an existing one if you
have one. The steps are the same.
Just a reminder, if you have your signature as you like
it here, if you also have one on your office computer in Outlook 
than you will have to
duplicate it there. One does not hold for both. They are two different
applications.