I have decent aim, but I usually die before I can line up a shot.
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I've also learned to check corners and keep track of flashbangs (still haven't figured out the avoiding part).
Greetings. Here is my 2 cents.
Regarding "usually die before I can line up a shot". The first thing you should know is that everyone has this problem. Even pros. The reason this happens often isn't because the other player is better at moving their crosshair onto you and clicking, but instead, they had to move their crosshair less or not at all in order to be on you. So, basically, you need to move your mouse and click while all they need to do is click.
Why do they not have to move their mouse? How is their crosshair already on you? The term is called "Crosshair Placement" and involves religiously keeping your crosshair at head height above the ground and keeping it in locations where a enemy is likely to pop out from. That second part requires some map/situational awareness in order to actually know where they are more likely to come from. That will come with practice on a map and in the game in general.
Again, the whole point of this is to reduce or eliminate the first step in shooting an enemy in order to make your reaction time faster.
Some of the Adren videos others have linked above are about this topic and show examples so I also recommend those.
Second, about avoiding flashbangs. All you need to know is that a flashbang will blind you if you are looking in it's direction. There is some minimum angle that you need to be looking away from the flashbang in order to not get totally blinded but I am not sure what it is. Might be 60 Degrees or something. You'll still get blinded, but not as much (can still partially see). If you look away further then that the effect will be even shorter, giving you the ability to engage the target accurately when they push after the flash.
Short answer: If you see a flash coming, look away from it asap. After it blows up, turn back to face the direction it came from since there may be an enemy charging forward who is under the assumption that you're blind. Show 'em who's boss.
Longer answer.... Ehh, I'm late for work. Will append later.
Topics to cover:
1. Juking flashes
2. Pop flashes
Have fun.
APPENDING TIME!
1. Juking Flashes
I have actually tried to find a reference source for this term, but could not locate one strangely enough. I have heard casters using it to refer to purposefully facing the wrong way in anticipation of a flash coming in, in order to be totally not blind and able to instantly react after the flash goes off, surprising the opponent. Not something you'd want to do as a terrorist (except maybe on hostage maps) but as a CT defending a bomb site this is sometimes useful. In a pub like TD though I wouldn't recommend doing it since people are more likely to just run in without throwing a flash then they would be in MatchMaking (MM).
2. Pop Flashes
This is when you throw a flash in a particular way in order to cause the flash to blow so quickly that the target has no time to turn away. Basically, think of this as playing Hot Potato with someone when you know exactly how much time is left on it. Sure, you could just toss the potato at them and hope they hold on to it for long enough OR you could hold onto it until it is about to blow and then toss it over. Also, if you played Day of Defeat the grenades work in a similar way. People in that game start the fuse and then hold onto the grenade for a bit before throwing it. In order to give the enemy no time to react when they see it before it blows up.
How do you do this in CS? Well, the fuse for the flashbang (and HE grenade) starts after you let go of it, so you can't just hold the button down. Instead, you need to throw the flash cleverly. Off a wall or off the ground or off a thing far away... basically bouncing it around so that it eventually goes where you wanted it to go, and it gets there just before it blows up. Learning these flashes isn't crucial for someone who is new to the game, but if you want to be using flashes offensively then this is a great way to increase their effectiveness. Just be sure to run around the corner right after the flash goes off in order to make best use of the time the enemy is vulnerable.
Thats all for now. I Hope this was helpful.