Step 4, The third cell...
The third cell is more of the same. First you need to sew the upperdeck & lower deck together. On the picture below the seam is indicated in "red".
Seam I - upper & lower deck
As the air inlet is triangular the upper and lower deck are only sewn together until approx. half the width of a cell (example is from cell 4):
Both of the edges of the inlet are reinforced with strips of mylar (is also possible with kevlar). These reinforcements will keep the inlet open (a little bit).
Seam II - Profile & lower deck
Secondly the lower deck and profile is stiched on cell 2.
Order: lowerdeck cell 3 - lowerdeck cell 2 - Profile.
Seam III - Profile & upper deck & D-rib
Thirdly the upper deck, profile and D-rib is stiched on cell 2.
Order: Profile - upperdeck cell 2 - upperdeck cell 3 - D-rib.
The end results looks like this:
