After setting the gable end rafter for the lower section of roof, the author measured down from the main ridge to the point of intersection.
After the scribed line was transferred to the outboard face of the wall, the top of the wall was trimmed to match the slope of the gable rafter.
The built-out wall I-joists also help address wind uplift because they are screwed into both the wall and the roof rafters.
The rafter was held in place to scribe for the cut where the lower rafter would intersect the main roof.
For a jig, two layers of OSB were cut to fit between the I-joist flanges; then another layer of OSB was screwed on at the correct angle to guide the circular saw.