1. Open internet browser
3. Click "Setup" in the top right
4. Type "signature" into search box and click the "Preferences, Personal Settings" result.
5. Click blue "Add signature" button (in the image mine says "View Signature" because I already have created one, but it should say on yours "Add signature")
6. Select the desired department from the dropdown
7. Type in desired signature
8. Click "Save" button in bottom left

Official steps from Zoho
here if these above are inaccurate.