1. Open Gmail.
  2. At the top right, click Settings and then See all settings.
  3. In the 'Signature' section, add the text of your signature to the box. If you want to, you can format your message by adding an image or changing the text style.
    • Tip: Your image also counts towards the character limit. If you get an error, try resizing the image.
  4. At the bottom of the page, click Save changes.

Source: Create a Gmail signature - Computer - Gmail Help