You're right, pics would help. But my wife is the photographer, and getting up on that roof once was once too often!
I used the full 6" width on the front and rear main seams, but cut it in half for a 3" strip around the vents. The skylight needed a bit wider strip to cover the seam sealer, so I used 4".
For the main seams, I wrapped it over the edge of the roof, about 1.5 inch over each side. For the vents, started on the downslope side, working around to the high side, kindof like doing roof flashing. But considering how sticky that Eternabond is, I doubt if it matters how it is applied. But I did try to cover the factory seam sealer completely, with overlap on both the vent/skylight/gelcoat and the roof membrane. That way you get the Eternabond seal on both sides of the factory seam sealer, plus the seal of that seam sealer as well.
I guess a picture would be easier than trying to explain it!