Go to the Signature tab, and create a new signature by clicking the + sign
Quit Mail (do not only close Mail, make sure you quit the program with CMD+Q). Go to your library by opening finder, choose "Go", and make the library appear by holding the "option" key on your keyboard
Go to Mail/V2/Mail Data/Signatures
Find the signature that was last created (the youngest one)
Open the signature file with Text-Edit
Open the file with your HTML signature in your web browser (Chrome, Safari, Firefox), right click, and "View Page Source"
Select everything in your page source
The trickiest part, when you replace the code in the file open in Text-Edit with your own code
Save and close Text-Edit, and lock the document.
You're done! Congratulations, you have a new HTML signature!
Installing a signature in Mountain Lion is a bit trickier than with Lion or Snow Leopard, but it’s still something you can do. You will have to follow these steps very precisely.
Go to your Mail Prefences
In the Signatures tab, create a new signature by clicking on the + sign
Quit Mail. This is very important. Do not simply close it, but really quit the program (CMD+Q)
Open Finder, and go to your Library. For this, choose “Go”, and make the Library folder appear by holding the “option” key on your keyboard
Go to Library/Mail/V2/MailData/Signatures/ and find the youngest signature (the one that was most recently created)
Right click that signature and open it with “Text Edit”. To do this, Open With / Other / select Text Edit
At this point in the tutorial, you’re almost done. You only have a few steps to go, but these are important. Leave Text Edit and the signature file open until I ask you to close the program.
You HTML signature is probably in a file that ends with .html. Open that file in your browser, right click anywhere on the screen, and ask to view the source. In Chrome, it is labeled “View Page Source”
Select everything (CMD+A) on the page and copy it (CMD + C). Also, have a look at your code, and make note of whether it begins with < div> and ends with < /div>
Go back to Text-Edit. Towards the end of all of the code, you should see a portion that looks like < div>Your Name< /div>< div>Your Email< /div>. This is the area where we will have to replace the signature with your code. If you code started with < div> and ended with < /div> (both conditions are required), then you can replace < div>Your Name< /div>< div>Your Email< /div> with your email signature code. Otherwise, only replace the portion of the code that begins with “Your Name” and ends with “Email”. Do not replace the < div> and < /div>
Close Text Edit, and get the information of the file you just modified (Right Click, Get Info), and make sure you lock the file. If you do not lock the file, Mail is going to reset the signature, and you will have to start all over again