Under the Email section, click Signatures.
Click the + icon in the lower left corner of the screen to add a signature. Double-click Untitled, and then type a name for the signature. This name does not appear in the signature in your messages, it is only for your reference.
If you just want to change an existing email signature, select the signature you want to edit.
In the signature box on the right, paste your new signature (or press Command + V). You can then set or modify the default signature settings by selecting the account and which signature you want to use for that account.
Once you are done, close all the windows until you are back at the main screen. Click to create a new email and check out your new signature!