You may have seen the send a plain text email option within a contact record > more options?
You may have also wondered how you add a personal email signature to these outbound emails?
A: Just go via your My Account > Personal details menu from the top of your screen and you will see the editor for adding in your email signature content.
You have an editor toolbar there to enable you to add text and an image. It is best to type the content rather than copy/paste from another source.
Once you have created your signature why not add yourself as a contact to your account (preferably with a different email address to your log in email address) and then send your contact a plain text email (via the More Options button) and see the fruits of your labour?! You can always then return to edit your signature further if you wish.
Hint: The send plain text email is ticked to add to contact history by default, so you know that these outbound emails are being auto-saved onto your contact's contact history, which is the first tab in their records.