Hilico,
You need three things for vortex ring:
1) Low or zero airspeed
2) Power applied
3) a high rate of descent; books vary, but probably more than 400 ft/min
Note you need ALL THREE of these. So if you get rid of one, you solve the problem. So all you normally have to do is move the cyclic forward and get some airspeed.
They say it's most likely to happen in a steep approach, probably to a confined area. However, most people are ready for it then, and IMHO it's most likely when you're concentrating on something else, eg at low speed or a high hover while a passenger tries to take photos or similar. But as The Nr Fairy says, you'll do it in training, probably quite a lot, so you'll get to know what incipient vortex ring feels like.
As for what happens...well, the root of the blade stalls, and the tip has no lift due to increased vortices...read the books if you really want to know.