There aren't many 'tricks' to playing riven. It's just mechanics that you practice and practice until it's muscle memory. And then after that it's learning her max potential, same as any other champion. It's just her mechanics to play optimally are one of the highest in the game.
Look up the fast combo. Boxbox as a video on YouTube somewhere. And then don't get tricked into learning 1shot combos. Those come naturally as you learn the champion and there are way to many to learn appropriately and all of them are situation specific.
That being said, once you are proficient with her basics, practicing combos in the practice tool will help you learn the fluidity of her combos.