any idea to how to get better at csing?

That's quite all right, after laning phase CSing becomes more of a macro problem rather than a micro skill. You'll just have to find ways to CS without being at the wrong place at the wrong time, really. This mostly comes down to experience, to judge each situation correctly and maximize the value you can generate for you and your team in each given moment. I can give you a few hypothetical examples, but those won't really help you all that much. I suggest you watch a very high elo streamer (top 50 challenger on EUW, NA or KR) and ask yourself at any given moment why they're currently doing what they're doing, and why they're currently in the place they're at. Ideally you want someone that's playing your main position and is trying to reach rank 1 (or at least top 10). This is very hard to learn as these kinds of decision making are usually an overlooked aspect of the gameplay since they're not flashy and usually don't result in crazy KDAs, however decision making and macro play can easily carry you into Masters alone, there's no more important part in league. For starters you should try to maximize your value, use your time as efficient as you can. 3 enemies pushing midlane? Whelp, your team should be able to win a 4v3 under their towers, try to grab some farm elsewhere. The enemy got 5 at dragon while you're 2 men down? Don't even try to contest that, better clear minions in a sidelane. You aced the enemy? Maybe not go farm wolves right now but push down that inhib.