First, a person is assigned a teacher role in the classroom, and then assigned to a course. The primary route from this card is through the teacher's lounge.
The role is assigned on the Teacher's Lounge registration page—after logging in, you immediately become a teacher. To assign a teacher to a course, open "Education" → "Control Panel" → "Teacher's Lounge," select the teacher's profile, click the "Course" button ("Assign Teacher"), and select the course.
You can also connect a teacher by going to the "Teachers" tab in any course and assigning a main or regular teacher there.