I'll put on a song, leave youtube autoplay going and just let it run into different songs. If something comes on I don't like, I'll alt-tab over to youtube and skip the song or pick a different one from what's on the screen.
Listened to the story of how this story was recorded, which was pretty cool:
Four of 'em sat around a microphone and recorded themselves singing C in unison for as long as they could, then went to C#, then D, etc... They looped each note onto a long loop of tape, had 12 tape players going with 12 tapes stretched across the room, feeding into 12 channels on the console. Then by sliding different channels up/down on the mixing board, they made themselves an infinitely sustaining Mellotron of sorts.
The ethereal sound comes from having all notes playing at a low level and providing an airy/dissonant sound in the background, but then pushing up the sliders to make different chords out of it.