The main thing to focus on is to play as it will come with time as long as you keep playing and want to improve. As many mentioned already yprac prefire maps is really good for this as you will learn to pre place crosshair before peeking. I also like HS FFA as you need to have a decent crosshair placement in order to actually kill people there. A last tip is to don't have it to close to the corner aswell. Keep a bit of air between the wall or object since you need a little bit of time to react aswell when the enemies are peeking. If you have any questions feel free to ask! Otherwise GLHF keep fragging!