Practise harmony, one exercise at a time.
Each exercise gives you one voice, a bass or a melody. You write the other three on the staff, hear the result on a piano, and come back to it whenever you like.
Arensky's 1000 exercises
Anton Arensky taught harmony at the Moscow Conservatory, where his students included Rachmaninoff, Scriabin and Glière. In the 1890s he published a collection of a thousand short exercises for the practical study of harmony: figured basses and melodies, graded from simple triads upwards, meant to be worked through one after another.
The book is long out of print and only exists as old scans. We are transcribing it exercise by exercise so you can work through it here, with your progress saved as you go.