Its kind of hard to describe. here are a few links
Essentially you will build your porch with a gable style roof with a ridge beam extending back to your roof. Cut individual rafters from the ridge to the roof to form a valley. Dormer framing is different because dormers are usually open to the inside.
I have this book
It shows what you want to do, sort of. Its a very technical book and trade oriented. For a small DIY project like you have your do not need to know all the math and techniques for production cutting large batches of rafters and so forth.
Good luck and take some pictures as you go