To send a message to an individual student, start by clicking on the "Messages" tab on your educator dashboard. 

Then to send a new message, click the note icon in the top left corner of the screen:

Type the name of the intended student:

Type your intended message in the box, and click "Send" or press "Enter" to send the message. The student will receive this message on his/her Overgrad profile as well as an email notification.