Cubed eggplant is a good cut for any dish cooked on top of the stove, like stews, ragouts, and stir-frys. Remember that the cubes will lose moisture and size when cooked. To compensate, make the raw cubes about twice as big as you want the cooked cubes to be.
Cut the eggplant in half or thirds lengthwise, depending on the size of the eggplant and how large you want the cubes to be.
Cut each half or third lengthwise again into halves or thirds.
Now just line up the spears up and cut them crosswise into cubes. (Some of pieces will technically be triangles instead of cubes.)