To piggyback on the answer of thesunneversets, look at how the pros play by watching their MTGO videos. You can find them on Channel Fireball and Starcitygames.
But there's a way you should watch them to get maximum benefit. At each decision point, pause the video and ask yourself what the right play is. This includes mulligan and sideboarding decisions.
Then, compare your play with what the pro does. If your play is different, try to figure out why the pro's play was better. Now, sometimes your play will be better but you better have a really good reason why.
To get more input on particular game states, take a screenshot of the game state and post it on MTG Salvation and ask them what play they would do.
If you do this type of practice a lot, you'll start thinking like a pro and then you can apply the concepts you learn into your game.
One thing though. This type of practice is draining and not particularly fun. It's much more fun to just keep playing games over and over again and hope that you're getting better.
By analyzing the pros' games, you'll be doing something out of your comfort zone. You will fail a lot and failure is not fun.
Consider the world class ice skater. She looks flawless in the Olympics. She makes difficult jumps look easy. However, if you were at her countless practices, you would see the hundreds of falls she experienced before she was able to nail those jumps perfectly. Falling down over and over is not fun.
You're going to have to have that same experience of falling down multiple times and learning from your falls before you actually become much better.
You can learn more about the process of improving skills by reading Talent is Overrated.
That book was super helpful to me for improving my game.
Oh yeah, one more thing. Regarding deck building, I would focus on technical play and just pick a tier one stock deck. Improving your technical play will lead to more wins than building a new deck.
Deck tuning is another matter. The only thing I would say about that is to know the card pool of the format and be mindful of the cards that are not pulling their weight as you play.
Once you've identified the worst cards in your deck, go through the card pool and try to find better alternatives. If you can't find any, then maybe the deck is just flawed or not good enough for the metagame. It might be time to choose another deck.