The peak is the tricky part multiply half of the overall wall length 12 by the vertical distance from the wall height to the peak 8 to get the gable end s square footage 12 x 8 96 sq.
Gable end siding shiplap.
A three person team two installers working from staging who feed measurements to another worker at a cut station on the ground can side a large gable end in an hour or two.
The corner trim gives a solid edge to fix the siding up to and it also gives some end protection to the end grain.
Measure between the trims at the top bottom and middle this will tell you if the ends of your shed side panel are square.
Gable end walls exterior is finished with lp smartside prepainted wood panel siding 4 8 sheets tyvek house wrap 7 16 osb attached to the 2 12 s on 16 centers.
Picture of corner trim as installed.
Then add 20 on the gable to cover the waste of all those angle cuts 96 x 20 115 sq.