Either:

• Open Marketing > SMS > Chat, click the customer in the left list, type the message in Write your message, choose the From number, optionally click Schedule to pick a date/time, and press SEND; or
• Open the customer's profile under Customers, use the Send Message window (From, To with the customer's phone, Schedule, message) and click send.

In both cases the message is queued and delivered by the background sending job (immediately at the next run if no schedule is set). The customer must have a phone number on their profile; the profile window shows "Invalid User Phone Number" or "Text message is empty!" otherwise.