Duck cloth is sewn on (top stitched) down the middle of the seat fabric. It tucks in between the seat back and the seat to secure tightly to the frame.
My particular piece was the width of the seat x 18" - divided in half at 9" - then sewn to the bottom of the upholstery fabric at 16 inches down.
Here you can see the duck cloth sticking out the crack in the back between the two seats - not secured yet.
Prior - with lining attached.
Close up of back of seat detail - red is darker than in picture.
Side gusset - upholstery is still loose from the seat frame.
Fitting the sewn seat upholstery.
#1 - The top of the seat and back rest (the largest square).
#2 - The accent piece (red upholstery fabric in this case).
#3 - A long strip for the sides (front) of the seat.
#4 - A long strip for the sides (top) of the back.
#5 - A piece of duck cloth the width of the seat by approx. 18" is sewn down the middle (at the 9" mark to the correct area and is attached to the back to slide in between the seats to secure the crevace between them to the frame - this is what will keep it looking upholstered and tight instead of a "slipcovered" look. (photos above about that).
#6 & #7 - Side gusset pieces (2).