The following instructions are for removing the stationary glass, commonly referred to as the vent window.

You do not have to remove the main window to remove the vent window.

1. Remove the interior door panel (instructions are here)
    
2. Take out the speaker at the bottom
    
3. Remove the bolt holding the bottom of the vent window to the door
    
4. Remove the two bolts on the front top of the door along with the mirror bracket
    
5. Lift up the vent window and giving it a quarter turn it will come out of the door.

Be very careful removing the vent window, especially if you have tinted windows. I learned this the hard way, as I chewed up the tint film on the driver's side main window replacing the vent window on my 91. The lower extremity of the vent window frame caught the inner surface of the main window on the way up and out.
-KDirk

* More information can be found in the body section of the service manual - section 10.5 in 88. GM calls it "stationary glass" and usually includes the front run channel of the sliding window.