I'm in the same process of practicing dense LN maps, where there really isn't much empty space. Anything I say is on my recent experiences and may not be the best way.
So IMO the best way to start is with the full LN map pack(this one:
beatmapsets/368373/#mania/819051) which basically has you holding down the keys for the entirety of the song. This was just so I can get used to holding down notes (you'd be surprised by how much your fingers can burn at first) and pressing immediately after holds. I haven't been on it for a while but I know I'm able to do lvl 1s and 2s on it.
From there, I try to find maps which the mapper is not afraid of using Long Notes. Now my main issue with the interpretation of the question is that I think you mean songs where LN is dominant with very little actual notes when most maps of such tend to use an even combination of both and happening at the same time, LN Hybrid.
If you feel that accessibility to LN Patterns for you is limited (I'm not quite sure of LN-Dense maps in the 2* range), do not be afraid or ashamed to use Half-Time. It's really useful.