According to what I'd read on the website, to get started I just needed to coat the terrain tiles with the appropriate glue, then apply the flock.
And from what I'd read, the recommended glue was something called Grass Slime:
A latex, aggressive hi-tack glue.
Although the manufacturer of the stuff - Noch - seems to have had a change of mind about the name: mine arrived with the same product number, but labeled "Graskleber (Grass Glue)."
The instructions were entirely in German, so I referred back to the website and learned that I just needed to brush the glue on.
So I put a tarp down (since flock is notorious for getting everywhere), laid down one terrain tile, and prepared flock, glue, and a disposable brush.
The glue container has a screw-off lid, and a pull-out lid under that. The glue itself looks (and smells) just like a batch of watered-down white glue - but is supposed to be "stickier" and stay sticky longer.
So I dipped my brush in the glue, and brushed it on to the tile. I didn't know how thick to put it on, so I just brushed the glue on (and around) until it was the same thickness all over.
I then flocked the board in my traditional, time-tested manner - I dumped a big heap of flock on top of it, and pressed down to make sure it adhered to the glue. The excess flock was then shaken off and put back in the container.
But for the second terrain tile, I had special plans...