Press "Return" to activate your cursor.
Press "CMD-A" to highlight the code.
Press "CMD-C" to copy the code.
Now go to "Mail", then "Settings", then "Signature". Press "CMD-V" to paste the code into your signature box, making sure first that the curser is in the top left hand corner of the signature box.
Click "done" on this page and the next. Now click on "write" to see your email signature.