RDinger, Wii's link is all you need to know, basically 'the rule of three' - allow three miles for every 1,000 feet of altitude. Usually you don't have to bother too much about 'rate of descent,' most aeroplanes are designed to follow that sort of descent rate with engines at or close to 'idle,' and FS models that pretty faithfully. That's equally true whether you're flying an airliner or a Cessna. In addition, if you have the autopilot on, the FS autopilots will automatically set a suitable rate of descent. In the FS9 default airliners, I think it's usually 1,800 feet per minute. Check at say the halfway point that you've lost at least half your height by that time; if not, increase the rate to suit.
Two more points that may help. Firstly, if you're flying an airliner at 33,000 feet, the 'rule of three' will tell you to start letting down 110 miles out. That sounds crazy at first, but it's right because of the speed you're travelling at; obey the rule!
Secondly, my ILS landing tute, which CD kindly linked to, doesn't actually cover the initial let-down - but it DOES tell you to get nice and low nice and early. Excess altitude's just a nuisance late in your approach, plan to get down good and early so you'll be well below the glideslope with plenty of time to intercept it.
Only thing is, check the map to make sure that there aren't any mountains near the particular airport you're planning to land at!
