Proper tables and relationships

I need some basic table/relationship help. Although I am not listing all of the fields and tables, I am hoping to get some general theory guidance on proper setup. The solution simply tracks courses that are taught and the costs associated and seems to work fine. Within a teacher layout, I have a dropdown to select the Step, which them populates the salary, and ultimately shows me the cost of the course in a Courses layout. However, what I failed to think about is historical data. As teachers move up the Step ladder and I change the Step in the Step field all data changes for courses they taught to the most recent step salary. I need a way to 'store' all of the Steps of a teacher over time and keep those values with the courses during that time. 

What would be the proper way to construct this? 

Tables: Teachers, Courses, PaymentSteps
Teacher fields: ID, Name,....
Courses: ..., ...., CourseCost
PaymentSteps fields: ID, Year, Step, Salary