If you want a simple Gmail Signature to follow the below steps
1. Open Gmail. In the top right, click Settings
2. In the "Signature" section, add your signature text in the box.
3. If you want, you can format your message by adding an image or changing the text style.
4. At the bottom of the page, click Save Changes.
Also; You can use a free tool like HubSpot that allows you to build a custom and professional signature