Harmonize an unmarked melody. Piano Pages   William Wieland
Play a tune with your right hand and cadence pattern chords with your left hand.

First, learn several left hand major cadence patterns from one of the following pages:
To get started, play the following pages:

Tunes which use only the 5-finger pattern
Familiar Tunes
Christmas Carols
Less Familiar Tunes

A middle school music educator once told me that adding chords to a tune was the most important piano skill she used in the classroom.