To send an SMS to a contact from a deal you simply click on the 3 dots next to the contact's name and from the menu select the SMS option:
After you have selected the SMS option you will be able to select a Message template and edit the Content of the SMS.
Once you are done with the editing click on the Send button and the SMS will be sent to that contact:
All sent SMS will be recorded as a deal note.
You can find instructions on how to create a Message template in this article.