You practice on bots in the shooting range or you can go with the cliche of using Osu. The best way is not to practice it, but to actually play the game itself. You can develop other skills besides crosshair placement when you play the actual game, instead of focusing on placement.
Another way to train crosshair placement is to go into a custom game by yourself and “smooth” around corners. smoothing is when you hold your crosshair head level on the edge of the corner as you are peeking. watch a youtube video on smoothing for more explanation. it definitely improve crosshair placement and mouse control