I'm gonna assume you can recognize good play when you see it, so with that said, look for bad plays.
For starters, make sure you toggle team vision back and forth to give yourself a better idea of what information was available to both you and your enemy, and think about any changes you could have made to that vision either through wards / sweeping / scouting abilities, that would have changed the outcome of any bad scenarios you come across.
Missing minions? That's bad. 80 CS in 10 minutes is a good target, and really beat yourself up when you miss a cannon. Try keeping track of how often you miss minions and in what scenarios, then work on your skills in a custom game if you need to.
Dying? Could you have positioned better? Did I place my ward in a good location to avoid dying from that gank? Was there a better place to put it?
Think carefully about why you are doing what you're doing at every stage of the game:
"I am pushing this wave in because I want to roam bot. I want to roam bot because the enemy bot lane is harassing under our tower, and I just saw the enemy jungler top lane."
-or-
"I am going to push this lane and back now because I have enough gold for a partial item and none of the enemy players are showing themselves on the map, so it's dangerous for me to go scouting alone."
You don't have to make the RIGHT decision in every single scenario, but you do have to MAKE a decision. Don't rely on instinct. Unless you are a 500k+ mastery one trick, instinct will get you killed more often than not, and even if you are, it's still often suboptimal.
When your bot lane got double killed, what were you doing? Where was the enemy laner? Could you have followed them? Is there a spot you could have placed a ward that would have alerted you that they were going for a roam?
Things like that. Everytime you see yourself make a decision, ask yourself "Why did I make that decision?" If the answer is not "because it will get me closer to destroying the enemy nexus" then 9 times out of 10 it's the wrong answer.
Question every move command, every ward placement, every ability, every item purchase. Question everything.