Here’s how:

1. Click Options in the upper-right corner of your Mail page and select Mail Options.
2. Under “Management”, click Signature.
3. Before you start typing, choose if you want to have your signature appear in Plain text or in Color and Graphics (click your choice above the compose window). With the toolbar that appears when you select Color and Graphics, you can choose which font you’d like to use and how you’d like to format and color it.
4. Now, add or change your signature by typing in the text area.
5. Check "Add signature to all outgoing messages" for your signature to appear on the emails you send. Anytime you’d rather not use the signature, either delete it in your Compose window, or to remove it for a more extended period, return to this page and delete the Signature text entirely.

Remember to click Save when you're done. This puts your new signature into effect.