WARNING: This page is about a different car, the 2002 Oldsmobile Bravada, 2002 GMC Envoy XL, 2002 GMC Envoy, and 2002 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Fig 1: Door Sill Plate (Rear-Long Wheelbase)
Courtesy of GENERAL MOTORS CORP.
Ensure that all 3 retaining tabs are attached to the sill plate.
Align the retaining tabs to the door frame slots.
Push down on the sill plate, seating the retaining tabs to the door frame.