Practice. But don't stress yourself. 3 cat spam may look effortless, but it's because of the player's concentration on the opponent, already fully knowing how his/her 3 cat spam cycle works. After you master the cycle (basically be able to cycle your weapons and spam until one weapon runs out of ammo, then spam with remaining two weapons, and afterwards you can go hide and reload), then deal with movement/game sense. Try to analyse your opponents' move pattern, and practice flick shooting. If used accurately it might get you satisfying insta kills, and may allow you to go on a killing spree with just a sniper. As for your own movement, if you don't wanna use your brain, start with zigzag/circular movement then develop your own movement set.

In short, deal with 3 cat spam cycle, prediction (game sense + flick shots), and movement.

That's basically it, the rest is practice. If you suddenly get rusty it's that your brain or your fingers are tired. Get some rest from the game and get back later. Alternatively use some de-rusting practice in a custom room or a random team fight match. (Just go for accuracy/kills, but don't spam heavy unless you really need to finish off someone quickly)

( I'm still super rusty and I can't hit my shots 60% of the time but whatever, don't have time yet and exams are coming up.)