Gables
In most cases, it's easier to assemble your gables on the floor deck in halves, then tilt them up into place and join them in the middle.
Start at one end, placing the panels near the base plate with the outside facing up. Apply caulk and adhesives on the splines, slip them into place, push the panels together and staple or nail through the outside facings into the splines. Once you get to the center, insert the integral post (that will be supporting the ridge beam) into the partially recessed edge of the middle panel and nail it off. Tilt the half-gable up on a caulked base plate, (drilled for wiring, if needed). After leveling, attach the bottom of the facing to the base plate, attach all the inner facings to the splines, and brace the half-gable in place.
Repeat the procedure with the other half. Just before you tilt the second half-gable up and slip it down over the base plate, be sure to caulk the integral post that will be used to both join the two halves of the gable together (plus support the ridge beam holding up the roof panels). Then tilt it up and pull the two halves of the gable together; you'll probably have to use a tightening strap to fully join the two haves at the spline.
Then, rechecking for level, work in as far as you can from either end, fastening the lower edge of the inner facing to the base plate. If you have a rough opening in the gable end, reach down and fasten as much as you can reach of the lower edge of the outer facing to the base plate. Finally, using a ladder on the outside, staple off the rest of the gable's lower edge of its exterior facing to the bottom plate. Working off the same ladder, fasten through the outer gable facings to the integral post; for particularly tall gables, you'll have to fasten the top portion from a ladder placed on the upper floor deck.
When the two gables are firmly fastened off, brace them carefully to make sure they don't go for a ride with any strong winds