How to Enable and Create Your Signature:
- Click your profile icon in the top-right corner of the page.
- Click your profile icon again in the dropdown menu.
- Select Preferences
- In the Preferences menu, click on Profile
- Scroll down to the Enable Signatures checkbox and click it. (This allows you to see other users’ signatures.)
- Below the checkbox, you’ll find the field to enter your own signature. (Type in your signature and save changes.)
That’s it! Your signature is now set up, and you can see others’ signatures as well. 
Additional Notes:
- You can create a signature without having to enable seeing other signatures.
- Based on my testing, signatures do not support images or GIFs.