Yes, it's easier said than done, but this is where practice comes in. But the trick is to exercise control while keeping yourself relaxed. So, where do you start? It's ALL about listening to the sound you're producing and CONCENTRATING! Of course, it's also about controlling your fingers, hands, arms, and shoulders. Not only one hand over the other, either you'll soon be working to make one NOTE out of a chord sound louder than the others, also as a common-place requirement for a good performance of many pieces.
I can tell you that after a lot more practicing, you'll find that making one hand louder than the other becomes not only easy, but necessary to playing many pieces correctly.
