I had a version that was a parody on the other tutorials showing up here. You know, the ones that are completely biased and have no merit? Yeah, those.
The Genre
Anyway, dark ambient (and ambient in general) is a very wide genre. The way I do things is tonal. (https://dl.dropbox.com/u/52803321/Glory ... 282%29.mp3 [Sorry for LQ]) This isn't exactly the more common version (quiet and noise) (https://dl.dropbox.com/u/52803321/amb22_r.mp3) but I like it.
Starting Off
The first thing you need, instead of VSTs, are samples. TONS OF SAMPLES. Don't have any? Record them. VSTs aren't exactly needed, with that track I posted being entirely out of samples. Samples are your best friend.
Wall of Text ("Fixing" Samples)
There's this neat little program named paulstretch (freeware!) to stretch samples and entire tracks. While I prefer to stretch normally, I make paulstretch stretch it for me with an emphasis on tonal. You can easily get an entirely different sound if you fool around with the variables. If I want to stretch it normally and then do a bunch of different effects (it really depends on what I want and I only remember what the names are when I'm doing it) in Adobe Audition. Audacity is the free-er option, costing nothing to however much Adobe stuff is. I just prefer the Adobe option, and I used Adobe Audition when it was at 1.5 (many years ago, now it's with Photoshop and that crap).
Got your samples? Do they sound weird yet? They sort of should.
Arranging the Thing
Place the samples in your DAW of choice. I personally use FL because I can see the waveforms, change it easily, and I've been using FL for 2 years now. From there, arrange several layers of samples however you want. Stretching it again to lower pitch and events are your greatest tools. Once you're done and happy with the sound, export it as a WAV. You're done now... unless you want to paulstretch it again for <insert reason here>. I personally don't do this most the time, but paulstretch does a good job.
Ending
The sound isn't as good as true dark ambient, which is made depending on what the author finds. There can't really be a guide. It's more of a "why do you make music" and "what do you want it to convey". Dark ambient answers a bit of both, in my opinion, but every artist has to find that out for themselves.
Good luck.