To be completely honest, you DON'T. You can learn a little from watching pro games, but there is a huge difference between when pros play and when anyone else does. When pros play, they play with and against the best players. Their mechanics are on point, they know the game much deeper than a typical player, and they have a different mindset. You can't learn that much from watching pros play because you play in a completely different environment. Sure, you can learn little tricks or ideas from pros, but that is about it. Watching a pro last hit is not going to make you better at last hitting. Watching a pro smite at the proper times is not going to help you get those timings down yourself.
Why do you think there are champions who's win rates suck at low elo, but then jump up at high elo? I would go as far to say that you could learn more from watching a friend at your skill level play and try to spot habits of the players in that game. Do you really think EVERYONE is going to play like a pro? They IMITATE pros by building the same items on the same champs, but they don't have the skill to play like a pro does.