Click on the "gear icon" in the top right corner and select "All settings" from the dropdown menu.
Next, click the arrow next to "Mail" in the menu on the left. Then click "Signatures". Click "Add new signature", enter a name for your signature, enter your details and feel free to edit it as you like using the editing toolbar below. Then click Save.
Tip: You can also add an image or photo to your signature. Just click the "Insert image icon" from the toolbar.