Let's say you've got the returns from your C/L assigned to a stereo pair - let's say - 40/41. You obviously have to create that stereo pair by SELecting CHannel 40 (hold finger on SELect) while pushing on CHannel 41's SELect key. The unseen magician in the console throws up a message: "Stereo? Yes? No? " Hit ENter and it's done. (There's an alternate way using a global menu but that's for another time.)
Now, Channel faders 40/41 are raised to 00 (unity gain). While those faders are in SELect mode, hit the MODule key. (Not necessary but it gets you dialed in for the final step). Next, look on your console for the 'Bus' panel (on the 3200 it's in the upper left/center quadrant). Find Stereo, and touch that button 'till the amber light is on. Now, 40/41 fader modules are routed to the stereo bus, along with any or all of the remaining elements.
Finally, inspect the details your 40/41 Module screen. Hit the 'SETUP' tab and look carefully at what's going on there. I can't tell you how many mixes I f***ed because something in that screen was awry. Examples:
-CHannel TRIM - this can be both handy and a knife in your back. Make sure it's sitting on 0 - with no ATTenuations (-3, -8db etc), or boosts (+3, +5 db etc).
-PHASE - here's another explosive waiting for you to step on it. Make sure either mono or stereo channels are IN phase --not REVersed (unless you did this deliberately for any number of reasons).
-AUX sends - If you have an internal effect assigned to Aux 1/2, you want these return faders UNassigned to those auxes. So Aux1/2 should be something like -100db ('off').
Hit the Module key and disable any EQ or Dynamics (at least 'till such time you think you might want to experiment).
A lot of the foregoing doesn't apply to the DM's 'crippled' channels - those without some of the described features. But if the other, fully active' modules are being used, everything above applies.
Probably way more info than you wanted but I had a few minutes.
CaptDan