I have been playing guitar for quite a while now, and I'm pretty happy with my abilities on the guitar so far. But of course, I want to become better.
Lately I've been listening a lot too more jazzy folk acoustic guitar music, think of Nick Drake or John Martyn, and I know that this is what I want to be able to play. An example of an artist is Ryley Walker, he is an absolutely amazing guitar player.
Here's a video where you can hear his skills and also see the type of music I'm talking about and want to learn. Ryley Walker KEXP

I want to be able to play in open tunings, and know the schemes so that you can just play around without playing a specific song. But how does one start? Like I said, I'm a kind of advanced guitar player, but only in the more standard and easier genres of music, of which you can fin tons of stuff on youtube and the internet. But this is a whole different direction, and I'm sure there are good sites or books on this but it's hard to find them.

I think there's someone on this forum who has some idea's on where to start.

Sorry for the long post, I am not a good storyteller and need a lot of words to say not so much, as you probably have noticed if you made it this far. Thanks
Everything old is new again......

This type of music, and playing, is very similar to what we were doing at the tail end of the "folk revival" back in the late 60s and early 70s. (actually, the 2nd folk revival... The first being in the 40s and early 50s....Folks like Woody Guthrie and the Weavers and all those folks.)
But anyway.... You could go back and listen to a lot of those folks since there was a whole lot more going on than "three chords and the truth".
However, I suggest a visit to Stefan Grossman's "Guitar Workshop" channel on Youtube. There is a very wide selection of styles and artists with lessons on early jazz, blues, slide, fingerstyle, etc, etc,.
Plenty to work from.
There are at least a few YT clips for any common alternative tuning (yep, alternative. is common nowadays). Tune the guitar and start experimenting and picking a few things from what you listen to.

Open tuning is really fun ant intuitive actually. As long as you think chord melodies and single note soloing. The only thing I'm using right now is lowering the A to G on my Tele and playing slide and some chords on it. It's like using a cigar-box guitar which has 4 strings tuned to open G. I don't have the patience to mess with tuning 3 strings every time to achieve open E and then go back to standard. However - open E is really fun - makes you get that big chord solo kind of playing.

Basic steps:
1. pick your open tuning (I'd go for open E before trying DADGAD or anything weirder).
2. learn a few open tuning Keith Richards licks - they are basic and easy... (if you need open G just capo the 3rd fret and use the lower 5 strings).
3. get a slide... try playing between strumming the open E chord and moving the slide over the other frets... it will get swampy quickly... then you'll get ideas for doing it without a slide (if you won't get addicted to slide work).
4. thing rhythm, groove and simplicity... this stuff sounds amazing and mysterious even without shredding or big music theory.
^^^^ Just a comment - I use 13s strings, and I never tune an acoustic guitar up, as it risks damaging the guitar. So I go down to open D rather than up to open E. - Open E would be OK with 11s, some guitars would be OK with 12s, and I wouldn't risk it with 13s on any guitar.

Bert Jansch was one of the early altered tuning pioneers. His version of "The first time ever I saw your face" in DADGAD is pretty challenging for intermediate players. - There are a lot of tabs available for his songs, but the ones for that particular song are poor.
DADGAD is tremendous fun. Open E is always fun too. Go listen to Duane allman, specifically "Little Martha", a great acoustic song in Open E if I'm not mistaken.