Learn about Harmonic Functions (teoria)
http://www.teoria.com/en/tutorials/functions/intro/
Practice Harmonic Dictations! Fun times!
https://msu.edu/~taggartb/courses/MUS282/exam%20practice/index.html
Understand Types of Contrapuntal Motion
http://openmusictheory.com/motionTypes.html
Probably don’t need to go into THIS MUCH DETAIL:
http://www.gmajormusictheory.org/Listening/progressions/Progressions.html