Playing easier stuff than what you can actually help a lot. If you want to improve your ratio and have never done it before, start off easy with 3 stars or something. Hitting harder than normal on stuff like jumps helps too, so you know if you're on beat or not. Don't hit too hard tho lol. HR is a good way to learning ratio, as it has an extremely tight timing window. Once you play HR for a while, you start to get used to it and that will help your ratio. V2 is also good for practicing ratio, as accuracy decreases when you get perfects. Knowing this, you can now try and improve your accuracy on the score with V2. V2 also changes the 320 timing window based on the OD, which V1 doesn't (if i remember correctly). If you want to practice ratio on other games, try out Etterna. It has way more stuff like rates, and setting your timing window by yourself. J7 is great for practicing. I recommend starting off with chordjacks/jacks to improve ratio, as they are arguably the easiest to do so. Once you get that sense of rhythm, you can go ahead and play more skillsets. Of course, you don't have to start with jacks, but it helps you kind of, feel the rhythm. Having an error bar is also good to have. I recommend scaling it up to 2x so you can see better. It tells you if you hit early or late, very useful. One thing I forgot to mention is that V2 also adds a release timing to long notes, so V2 is good if you wnt to practice just that aswell. And don't worry if your accuacy doesn't improve quickly, it takes a kot of time. What I'm about to mention next has nothing to do with improving ratio by itself, but it'll help if you don't have what i'm about to say. Your keyboard. Having a 1000hz keyboard is a big advantage for ratio. It registers keys more quickly kind of? Whatever, the point is, it helps a lot for ratio. Some people say it doesn't help much, but it really does. Take me as an example. I had the worst ratio known to mankind with a 125hz keyboard (even getting bullied for it), but after I switched to 1000hz, my ratio became extremely good. Like, really good. Especially jacks, my god.
Anyways, I hope you spent your time reading all of this, I know it's a lot, but trust me, it'll help.
And I know it seems kind of ironic so much coming for ME, a person known to have bad ratio, but hey, my ratio has gotten way better. Like, way better. How did it get way better? By doing the things I mentioned above. Remember, practicing anything requires patience, and without patience we don't have anything. Good luck!