I'm tired of these kind of questions as hotmail often change user interface. From hotmail.com to live.com, then now outlook.com...
In Jan of 2016, the user defined live.com signature suddenly disappear. After a browse, I found it was changed again...
Anyway, back to the topic.
Step 1. Look for setting at top right corner.
Setting -> Options -> Formatting, font and signature
Step 2. Edit your signature and save.
