I only play 4 keys and my LN skill are just average, but I'll give out how I built my LN skill until now.
TBH, the first thing I did was getting a skin with a nice longnote end (or whatever it's called) so I can read it better. If you have trouble reading this will help you.
LNs, they're really different and I suggest starting at lower level LN maps. 3 stars would do, tbh. Don't force youself with LNs that are too dense/heavy. Just start playing more LNs until you get used to it. Just play like you are a new player, increase the difficulty slightly every now and then. After then, you could start playing the LNs that are spread between all 4 keys, Zen Zen Zense and Blue Zenith are some examples. And there you go, you're now atleast not horribly bad at LNs. And tbh, after that I have nothing to say, because that's where my current LN skill is, and I don't wanna spread it out and act like I know what I'm saying- hope this helps even if just a little.