Alright so between watching pro players (either by videos or streams, streams are usually better because they are always current) or practicing.
First of all let's talk about why it is good to watch streams and later we'll get to how to use streams to your advantage.
Pro player streams basically show you how to play the game and how to play a champion. Basically the entire match the pro is playing is an example of what to do in the given situation. Pros love to test the max ability of champions, it's how they learn the limits/strength of said champion and often if they die will basically monologue why they died and what they should have done that might have ended with them living. Pros will often use little tricks that they know for a champ:
like Jayce Q then E so it's harder for the opponent to react. Aside from these little tips/tricks you get to see when the champ is strong or weak, what items to build and other general/basic knowledge of the champion being played
You also get to see strategy applied to the game , base racing, build orders, pushing when you laner roamed or roaming as well, ward control. You get to see all of these strategies applied to the game by players with high amounts of game knowledge. For example you might learn when it's appropriate to contest baron or give it up and push an inhib.
It's not enough to just watch these streams if you hope to learn anything you need to question what they're doing. Ask yourself questions like:
Why did they decide to base?
Why did he push?
Why did he roam?
Why did he ward here ?
Why did he trade now but not earlier?
Why did he build this?
And so on...
Once you begin to understand why pros do something you can start applying the knowledge to your own play.
As for the practical side of things. This, if you plan on watching streams to learn, should come after. This is when you practice farming, combos or strategies you learned when watching a stream. Couldn't quite get that leblanc combo you saw on stream? Well now you practice it until you can.
Of course you don't have to watch streams before practicing, you can learn anything you would learn in a stream on your own, through your own practice. And some things like farming cannot be learned through a stream and only improve with time and practice.
In the case of farming, spend you time in a custom with no bots. Select the champion to learn their AA animations and use no masteries or runes for a harder practice. Farm till 10 mins and see how high you can get your cs, I'm not gonna tell you a bench mark because like almost everything in LoL how good you are doing is relative to the enemy, 50 cs at 10mins is better than an opponent with 15 cs.