Gable roof help

[magneto259]

Hello all I'm getting ready to put a new roof on my existing pretty much flat pitch room that used to be a porch. I have some friends that are knowledgeable and willing to help but I was wanting to get a headstart and get the posts and joist hangers installed.

I am going to put full length rafters the same width of the house over the existing room and gable it into the existing roof. I have a friend who built houses for over 14 years and he's going to show me how to cut the rafters and such.

I'm also going to remove the flat roof over the room when we get started.

What I would like to know is how to get the 2 posts on each side of the room that will support the headers in the right spot, straight and square. I'm going to use pressure treated 4x4's.

Also where do I place the 2x hangers on the existing walls to hold the joists up? I know i'll have to remove the siding or cut some out but I don't know the
way to get the exact location. I imagine it would have to be in alittle from flush on the side of the house so you can put something over it such as a fascia board and it don't stick out from the side of the house.

Heres some pictures.

Thanks in advance all!