Good evening Herefish.

I might be able to help clear the fog for you,

"I have been looking at youtube for how to do a snakeroll, but they are all so fast,"
Thats normal, it looks fast, because the line in in the air going around and around, if not fast, it will fall in a heap around your ears

" I can't get the drift of it, do you start with a roll cast,"
Wrong, you finish with a roll cast, single Spey.

The cast basically is.
Tight line to reel,
rod tip low, near the water is better, more so will a sunked line.
lift rod to about shoulder height, the tip that is.
now without stopping, which will cause loose line and kill the loading.

Start to write a lower case letter e in the sky, starting at the inside of the e letter.
wind the rod tip around,
as the rod's path comes around to the bottom of the e, lift the rod back into the position to form a r/cast or single Spey, the Dloop behind the caster.

That easy !
And now the trick, if you could call it that, I think of it as the foundation step.
With a Double, left hand on the lower grip, hold this close to your body, as if to lock it there.
Once you master the cast, this locking of the lower hand doesn't matter too much.

"then cast it downstream, then up? "
?
cast across the stream, drift the line and fly down stream, snake roll and reposition the line and fly again across the stream.

'What are the anchor points they talk about.'
this is only the water's grip on the line, leader and or fly, which inturn helps load the rod.

this is why Spey casting on grass is near on impossible.

Another helping trick,
practice with just a bare rod, till you feel comfortable and can controll the size of the e that the rod tip is forming.

Kind regards,
UB
ps, another trick, don't try with a 15foot rod inside the house, more so with a ceiling fan