My suggestion would be that, at first, while you get to know her, don't pay attention to Luma. Focus on Rosalina only so you get used to her attacks' speed, knockback, range, damage, and stuff. You'll be more aware of her attributes overall: running speed, falling speed, grab game, throws (Back Throw is specially good if the opponent is over 90% and you're near the edge), etc. Her jab game is also very good.
After you know how Rosalina plays, you can start playing around with Luma, and do the same you did with Rosalina. Luma is very strong, and obviously not as complex as Rosalina.
That is the way I learned how to play as her. She looks like a complex character, but she isn't really thaaaat hard to master. It just takes time and dedication. Feel free to ask us Rosalina mains for any tips if you want to know more