Etienne has a good video on improving in another game called 'Etterna', but the same principles can pretty much be applied to any 4-key rhythm game such as FFR. Here is the video:
https://www.youtube.com/watch?v=jz-b7qo9nyo
However, one thing is that this video is a bit more advanced so it might seem a bit overwhelming at first-- but your own pace is fine. Feel free to return to it whenever you feel more comfortable over time
From my own experience, these are the three main things I needed to do in order to get better (some of which are also mentioned in the video)
1) Play a lot
2) Be willing to expand outside of your comfort-zone, but not so much that it isn't playable for you
3) Also be willing to make mistakes and be patient with yourself. Progress will happen over time
When initially starting, the hardest thing is probably pattern recognition and reading. Try spending time playing lower-difficulty files until you feel comfortable seeing and hitting the proper notes, and build your way up from there. The two main things you need in order to develop skill are pattern recognition and muscle memory. Don't be discouraged either if progress feels hard, you can do it!
I would also try to find a layout you feel comfortable with. Generally, players gravitate towards a "Spread" layout rather than using the arrow keys. So a spread layout might look something like this:
- Left: A
- Down: S
- Up: K
- Right: L
Last thing I think is important to mention-- learning where to look and keep your eyes is imperative to being able to read the arrows. New players may often develop a bad habit of moving their eyes to each individual note that comes up, but as you get better at playing, your eyes will generally be pretty stationary. Just try and find what is comfortable for you.
(Also-- I don't think I'm the most qualified person to answer this, but I hope anything I said helps. Cheers! Hope you enjoy your time here)