On the hardware side, you need a working Thrustmaster joystick (and optionally, throttle and rudder pedals). I assume you already have this.
You'll also need the following:
You also need a gameport to plug the controls into. If you use Windows XP, then this is very easy: buy a sound card that has a well-supported gameport. I use a Creative Labs Audigy LS, which is an older card that works beautifully, (check Ebay other other online retailers), and I've used a Sound Blaster Live! that works fine too, but Creative Labs sells current cards that have gameports as well.
If you have Windows Vista, this option won't work (apparently), since Microsoft has decided to drop gameport support for its newest OS. Only time will tell if drivers or patches re-enable it. I'm guessing someone is going to make this happen eventually.
You can also get a USB-to-Gameport converter, and these work in Windows Vista. I've already tested one.
Radio Shack used to sell catalog number 26-164 "Game Port USB Adapter" that works pretty well (check Ebay or check your local store shelves, where it might still be found). It has a 4-position switch to make it compatible with different controllers. You want to set it for position number 2, labeled "Joystick A" on mine.
There's also the "Rockfire USB Nest Joystick Converter," but I've heard scattered reports that it makes the the throttle jumpy (but I don't know for sure.) These are very widely available, but I haven't tested one yet. If you like yours, please let me know.
The "best" item is a do-it-yourself converter kit called a Precision Joystick Controller BU0836, available directly from its creator, Leo Bodnar for only $30. It is, however, a bare kit, and it is up to you to create a full converter from it. Also, as I see it, you'll only be able to use it to convert joystick movements, and not button presses unless you're willing to completely rewire your joystick and throttle, so you'll still need the keyboard connection. However, it has built-in movement smoothing, so jumpy pots should be automatically smoothed out, which is a huge benefit. If you go this route, you'll need to wire it all up yourself, but there is information out there on how to do this. Additionally, you can use a free macro program called AutoHotkey to simulate a keyboard keypress when a rewired joystick button is pressed.
You also need a standard PS/2 keyboard port, or an old-style, 5-pin AT keyboard port plus a keyboard adapter that actually works correctly (many of these adapters are cheap and nasty, and don't work -- look for a "pigtail" adapter instead of a "dongle"). Though these are, as of late 2006, considered "legacy ports" by Microsoft, Windows Vista still works just fine with them. The latest and greatest motherboards, however, may lack these ports, so when buying a new PC, be sure to check for them.
There do exist USB-to-PS/2 adapters, but these may not be a wise idea to use. I have a WCS throttle that died soonafter trying one of these adapters, and I don't know why. Maybe the throttle was about to die anyway. Maybe it was a bad adapter. Maybe there are levels of compatibility with these things I don't yet know about. But regardless, there's no way this adapter would "pass through" downloaded programs into the stick, in DOS or anything else. You could theoretically use this adapter (assuming it works correctly) to use a pre-programmed stick in XP or Vista, I guess. If you manage to get this to work, let me know.