When I'm cooking at home, I almost never blanch and shock (that's the ice-water bath). No need to, since I'm making the meal at that time and don't need to hold the veg. (Only exception is broccoli rabe, which I'll blanch to remove some of the bitterness when it's really strong. And even then, I don't shock; just drain well and toss in into a pan to saute.)
At work, though, it's standard practice to blanch and shock, especially veg like asparagus or green beans or carrots. Usually, as much as you'll need for the entire meal service -- typically several hours. We do it in restaurant kitchens so that we'll be able to put out the food faster, but still fully cooked and beautiful. What blanching/shocking does is cook the veg part way, and then stop the cooking process so that the veg is not overcooked in the end. It also sets the color so that it will still be bright and delicious-looking. The idea is that you will have it ready to "finish" -- that is, do the final cooking with its butter or sauce, or in the dish you're adding it to at the last minute. I guess if you were going to serve them "steamed" you could just drop them back into some boiling water until they're heated through.
The "how" of blanching is a lot like cooking pasta: lots and lots and lots of water, salted for flavor, at a rolling boil. (Lots of water means that it will return to the boil fast.) Dump in the veg. When time is up -- that's the tricky part -- scoop out the veg, or drain in a colander, and dump them in a big bucket of ice water. Swish them around until they're cold. Pull them out, drain them well, and put them in the container you'll use them from.
The timing depends on the veg. It takes practice and a lot of testing and tasting. You want the veg underdone, but still somewhat cooked. If you're doing it for yourself, just practice until you're satisfied. If you're doing it at work, have your chef or sous taste and tell you what's the desired degree of doneness.
Hope this makes it a little clearer.
