Practice keeping it within a tight range. I generally shoot for plus/minus 10w and can keep it pretty steady over undulations. Maybe start with a bigger range and work on making it tighter. It took a bit of focus and practice initially. I also found that the harder you're going, the easier that is to do. It's eaiser to stay steady at threshold than easy pace.
I also disagree that it has anything to do with a smooth pedal stroke or putting pressure all the way around the pedals. That's not how anyone actually pedals, and not as efficient anyway.