What is the general procedure for descent?

I have been using both the built-in ATC and Radar Contact and I am kind of confused as to when I am supposed to enter a STAR and select an approach in the FMC.

The built-in ATC seems easier to work with.  It starts stepping me down (I use cruise descents) and then assigns an approach say 60 nm or so out from the field.

Radar Contact, on the other hand, flew me all the way down to the initial approach altitude and turned me to a localizer intercept heading before even telling me I was going to fly an ILS approach.

In either case, though, am I expected to have this all programmed into the FMC based on ATIS info?  In the case of the built-in ATC, it assigns the approach at the exact same time I start receiving ATIS and it has already stepped me down 10-20 thousand feet.  So where is the top of descent in this case?  How would I know ahead of time which runway ATC will assign in the case of multiple active approaches?

Radar contact will simply vector you to the active runway.  You will be given an opportunity to select "the full instrument approach" using the RC menu.  Then they will leave you alone so you can execute your published procedures.

You can even pick your own, although you might want to consider wind and the presence of other airplanes coming at you from the opposite direction.

Alternatively, you can plan out the whole thing in advance, right down to which runway you are going to use and your SIDs and STARs.  While this is not strictly realistic, I consider it a legitimate concession to the fact that I am flying the airplane all by myself without a first officer to share the workload with, I don't have the same degree of information flowing to me along the way, and canned ATC can be wonky sometimes.

You can even change the wind direction by individual weather station via FSX, so you can be sure of getting the approach you want.

If you have planned everything out ahead of time, you can use RC if you want, but then just pick your runway and tell them to leave you alone (i.e. full instrument approach).  Or just forget ATC and concentrate on mastering the equipment and procedures like many tutorials do.

I hope this helps.  I you have any other questions, please don't hesitate to follow through here.

I'm still having issues trying to figure out the FMC for descent.  I have basically stopped worrying about ATC for now.

I made a flight from Tampa to Dulles and used a STAR that terminated at an intersection where vectors to final would be provided.  When I selected the runway, STAR, and transition, everything worked great.  The FMC calculated the T/D, E/D (the fix rather than runway), and the altitudes at each fix in the STAR right down to the final altitude where vectors should be provided.  I gave myself my own vectors from there...and had to just execute a visual approach because the FMC had the wrong ILS frequency for the runway.

I tried another flight from Portland to Los Angeles and did the same thing: picked a runway, STAR, and transition.  This time, though, the FMC just left all of the altitudes equal to the cruising altitude and did not calculate anything.  Modifying the altitudes manually got me there, but the FMC still did not have a T/D or E/D.

Do real FMCs do this?  Is it just a bug in the Captain Sim FMC?  Is there some way to handle it?

The end of my original post was never really answered either.  I am just a lowly private pilot / instrument, so I am not used to contending with a FMC.  When flying the big rigs, does ATC step you down before assigning an approach?  Is it important to have the FMC programmed before you begin stepping down?  If not, where is the T/D?  Do they give you more advanced notice than they do when you are puttering along at 110kts?  If not and you have to have everything programmed in before descending, do you just guess a runway if you have ATIS and parallel approaches are in use?

I am probably just thinking too hard about this.

You have good questions, and I think you are just butting up against the limitations of canned ATC, even the good stuff.  The advantage of RC is you can pick your own runway.

In real life, pilots have a good idea what runway they can expect, and can plan accordingly, but they may indeed get changed over.  Hardly any simmer thinks about it, but pretending to fly a 757 all by yourself and calling it "real" is a little silly.  I usually plan everything out in advance, including my approach and landing.  That is more fun for me than dealing with the problems you are describing.  Once I'm perfect at that, I may throw more at myself, but the idea is to have fun.

In Radar Contact, one of the options is "Req IAP Approach". This allows you to fly the full SID without much interference from ATC. You will only get traffic calls and updated barometric pressures. It is up to you to get to final appoach at the altitude assigned by ATC. Once you are established on final, you will be handed over to the tower. Unfortunately, this option only becomes active within ~60 miles of the destination. So you may need to ask for a PD (pilot's discretion) descent, or just descend at 500 fpm or so until you are handed off to Approach. If things go wrong with the SID, you can always ask for the Vectors or Visual. Refer to the RC Manual, starting on p. 54.