You can add an instructor to a particular class in two ways.
1. Add instructor via schedule page
Goto schedule/reservation
Click on the add schedule button on the top right corner.
In the time slot, select the instructor as applicable.
2. Add instructor via the calendar.
- Goto calendar/reseravtion
Then select the particular class from a specific time slot
Then click on the edit icon from the popup, and you can change the instructor as per the dropdown and change.