I really like doing this sort of thing, but I agonize over the littlest things.
The hardie specs say to use 16ga 2" finish nails for the fascia and trim. I couldn't find these anywhere so I finally googled it and HD and Lowe's sell them, but only for a finish nailer. Don't really want to buy a finish nailer so I figured I'd just get a box and bust them up and use a hammer. But, sheesh, those are tiny little nails. I can't see how they could possibly hold this stuff up. I ended up buying some 6d 2" nails and even those seem tiny. I dunno. And, they say to nail the siding with roofing nails. That doesn't seem right to me either but I'm going with it.
Then, I drove around and looked at roofs in the neighborhood. When I was young, we put down valley metal in the valleys and shingled on each side and cut about a 4inch path for the water to run. But, apparently, nobody does this any more. They lay one side over the other and cut the shingles on the high side so the water just goes down a shingle valley. So I went online to see how they do this and they showed some sort of water barrier stuff they put down in the valley first. I got some of this stuff.
http://www.owenscorning.com/roofing/...-self-sealing/ Not sure if I'm doing it right.
also, here's everything I bought today (hope I'm better at roofing than taking pictures)