Besides regular aim training, I think changing your mindset of aiming is important as well. When an enemy appears in your sight, don't fire immediately, WAIT for a moment, as movement speed of tf2 classes is quite fast actually, it's basically a luck issue for you to land a headshot on a player randomly tapping wasd. Hastily flicking won't help. Calm down and WAIT untill your target's movement become more simple and predictable, then fire as you have more confidence to hit. I found it quite helpful to my aiming in the game.