The Quick SMS subsection in Send allows you to create a one-off SMS message quickly and schedule it for delivery to one person. First, make sure you have the SMS credits before sending a message.
Navigate to 'Quick SMS' under the Send tab in your account. This opens a page where you can enter your 'from' details, the number you want to send to, and your message.
Once you have entered your message and chosen the time you want this to send, click 'Send SMS' at the bottom, and the message will be sent.