On a laptop/PC, click on your username near the top right corner of the screen. Then go to your Account Settings.
If you're on a mobile device, tap on this icon:
EDIT: Oops, I couldn't find an option to add a signature on a mobile device. And they don't seem to show up when viewing the forum on mobile.
Once you're on the Account Settings page, there will be a menu on the right with a Signature option: