Greg,

From Top Of Descent, you need to get to somewhere 12 - 20 miles from the threshold of the runway on an intercept course to the extended centerline at about 2500 feet, or at any rate below the glide slope, and at a reasonable speed, say no greater than 210 knots.  How you do that is up to you.

You can follow ATC.  FSX has it, or you can use a superior third-party program like Radar Contact.  Either one will get you to where you need to go.  Or you can use a procedure chart.  That is a STAR picked from your arrival page in the FMC.  You will want a seamless transition from your route to your approach.  It will have a transition, which should be the very last waypoint on your route.  (It helps a lot if you have the latest Navigraph nav data and charts.)

You are wanting to intercept the localizer at something less than a 45 degree angle and below the glide slope at a speed of VREF 30 + 40 knots.  (You find your Vref 30 speed on your approach page of your FMC, then just add 40.  It will probably be somewhere around 180 knots.)  I have my flaps at 15 at that point, ready to go to 20 once the glide slope is intercepted and the gear goes down.

You will probably be controlling your direction of flight by the heading knob.  That's fine.  If you have LOC pushed, it will automatically override it and steer you toward the runway.

It will do that if, that is, you are tuned into the localizer and have the proper course dialed in.  You can get the information from your approach page on the FMC.  Actually, it should tune your nav radio automatically, but you'll want to check it on the console to make sure.  (There is no law against manually doing this, either.)

Now, imagine you are just before the point described above.  You are coming in at 180 knots, at 2500 feet, well below the glide slope, on an intercept course for the localizer.  You have heading engaged, the LOC button pushed.  Flaps are at 15.  You have previously checked to make sure your nav radio is tuned to the localizer.

Just before you cross the extended centerline, the autopilot should start doing its magic and the airplane will turn toward the runway.  At this point you are still well below the glideslope.  If not, you should have started further out!  Once you are established on the localizer, hit the APP button.  This will allow the glide slope to be captured.  Eventually you will see the glide slope indicator start to creep down.  When it is approaching the middle, go ahead and add 20 flaps and put your gear down.  Keep your speed steady at this point.  You SHOULD begin to follow the glide slope down now.  

Now lower your speed to your VREF + 5 and extend to full flaps, making sure your approach is stable.

Typical gotchas are coming in TOO HIGH, or TOO FAST or, most likely BOTH TOO HIGH and TOO FAST!