I am not one to really help here as I am still a beginner myself, but from what I understand with my builds is: there is an angle of the template that is pressed against the root rib (stand the template up on its edge to do this) and hold the angled edge according to the plans instructions against the rib and press the rib into the template angle (so that the rib is leaning onto the angle of the template) and when the rib is leaning into the correct angle glue it in the proper places according to where the plans tell you to.

That is the basics, I am sure someone here, such as RCKen, can explain it more detailed.

My opinion, and how I do it, is to trace the dihedral angle template onto a piece of SCRAP WOOD, and then cut the wood into the template shape. The wood is more sturdier than cardboard and less likely to fall over or bend while doing the dihedral rib.