Hope the writing's going well, Gus.
The blades are generally tied down when the machine's left for a decent length of time - if I'm going to be away for more than a few minutes, I tie them down.
It depends on the wind conditions and/or the likelihood of other helicopters operating nearby - the aim is to stop them being blown about. If I'm shut down in some remote pad, the wind's light and the blades aren't moving, I'll just leave them.
The Bell 206 has a 'teetering head' - the blades are free to rock like a see-saw, up to a few feet each way at the tips. In strong, gusty winds they can really bounce around, putting stress on the gearbox mounts, so we tie them down.
As Whirls said, we use a hook that goes into a small lug with a hole in it that sticks out from the end of each blade. You put the hook in, then lead the blade around until it's aligned fore and aft, and you're standing next to the rear of the tailboom. On most machines there's a little tie-down point on the side of the tail boom that you then tie the hook webbing to.
Conversely, when you start, you must make sure the blades aren't still tied to the tail boom!
To do this, most people untie the blade and walk it around to the side, or slightly in front of abeam and take the hook out. That way, when you hit the start button, you can watch it from the cockpit and be sure it's not still tied down. I put the blade I've walked around out to my right and slightly in front of abeam.