First, select the messaging tab then select the create icon in the bottom right hand corner. Now select your recipient by typing their name or number in the "to" section at the top of the screen, now type your message . When finished simply press the green icon on the right hand side. N.B. to send a Delayed SMS, hold on the green icon and select when you would like the message to be sent.
...It’s basically the same as you would normally text on your phone.