Hey, help is what we are here for. It thrills me to see a question that I have dealt with in my years; then I can help someone.
I would cut the octagon pieces with an 11* miter on each end of the face part and back part, cut the bottom of the channel an inch or so longer so that each one can overlap the one below it. Then caulk the J before installing the siding. Think of roof shingles, each one has to overlap the one below it or the water would be funneled in.