Under the settings tab, in the email to SMS section, simply click on the email address you'd like to add this feature for (or enable a new one, if required). Then, make sure the "Include subject line as part of the SMS text" checkbox is ticked.
Your SMS will be made up by the subject line of your email, followed by a whitespace, and then followed by the body of your email. If you just want to use the subject line, leave the body of your email empty. Note that the subject line will count towards the 160 character limit for SMS.