Planking for CapeCod Ceiling?

HomeDepots' Pacific Knotty Cedar; 6 pieces to the package; 5/16" thick x 3.5" wide x 8' long; installed coverage 14 Sq ft.
Iam installing this on a CapeCod ceiling/(actually roof) (and, yes, i know better, but that is a long story).
The ceiling doesnot come to a peak, but is flat for 3 and1/2' down the center (side walls are 39").
This is going to be the ceiling to this bedroom. Iam installing it in the manner of a porch roof, on to 2 by 6, ceiling rafters to rafter (there are bats of fiber-glass insulation behind these fragile planks). There is a 2 foot space between each roof rafter. What is the best way to do this?
Thank you for any help at all.
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