Some club players chop at the ball when they attempt a sliced backhand approach shot and, as a result, knock it into the net.
For a smoother, more fluid stroke, imagine that you're moving your racquet head steadily down a playground slide as you swing to meet the ball. Start your racquet head above the level of the planned point of impact and let it slide down, hit the ball and finish below that point, out in the direction of your target.
This high-to-low forward stroking motion, combined with a good weight transfer to your front foot, will combine to produce a shot with underspin. Your opponent will be forced to hit a rising shot, which you can pounce on to end the point.