To create a signature in Mac OS X Mail:

1.In the Mail program, Click on the Mail mail menu and look for "Preferences..."
2. This will bring up the Preferences panel.
3. Click on the Signatures icon.
4. Then Click on the Create Signature button.
5. This will bring up panel in which to create your signature.
6. Beside Description:, Type a good, descriptive name for your signature.
7. In the box beneath Description:, Type your signature as you would like it to appear in your messages.
8. You may drag an image into the box and it will appear as part of the signature.
9. When you have your signature the way you want it, Click on the OK button.
10. Click on the up/down arrow beside Select Signature, then Click on your signature file.
11. There are three options regarding how to apply signatures

• Adding a signature to every outgoing message.
• Choosing which messages get a signature.
• Let mail choose a random signature from several you have created.

If you want to choose when to use the signature, Click on Show signature in composing window and see Adding signatures to individual outgoing messages, below. (If you don't check this box, this signature will automatically appear on every e-mail message you send out.

Adding signatures to individual outgoing messages

• If you have chosen "Show signature in composing window" A popup menu will allow you to choose from signatures you have created.
• If you don't want to include a signature, select None.

If you wish you may create several signatures and let mail select them randomly.

• Select Mail | Preferences... from the menu in Mac OS X Mail.
  
• Go to the Signatures tab.
  
• Under Select Signature:, make sure Randomly is selected.