This is probably the most important thing to work on. You need to guide your crosshair into positions where you expect the enemy to be. If you’re going around the corner but your crosshair isn’t pointed at a possible location of an enemy, you can lose even a second of adjustment time that will mean a difference between life and death. The time it will take you to aim and swivel might make you lose against better players.
Know your angles, know your crosshair placement, and shave off the time it takes to acquire a target. Think about where your crosshair is at all times.
Crosshair placement is essential, then comes the reaction speed and the flick shot accuracy. so practice, practice, practice!
A good way to start practicing for free is 3D Aim Trainer. It is a great aim training platform for CS:GO and other FPS games. You can sync your sensitivity settings from the game and practice playing the exercises for reaction speed and flicking.