These instructions assume that you have an HTML file that contains the email signature you wish to install.

1. Choose Mail > Preferences, then click Signatures.
2. In the left column, select the email account you want to use the signature for, then click +
3. You can drag signatures from one account to another, or to and from All Signatures. If you create a signature in All Signatures, you must drag it to an account to use it.
4. In the middle column, type a name for the signature.
5. The name appears in the Signature pop-up menu in the message header when you write a message.
6. Open the html file in Safari (not another browser), use the command Command-A to select and Command-C to copy the design to the clipboard.
7. In the right column (the preview), create your signature by pasting in the copied design (Command-V).