This is about as tricky as it gets. You need to know how to demo and resupport the roof rafters, frame complicated angles and work all the right flashing etc. This is a job for an experienced carpenter. Most carpenters aren't even good enough to do an a+ job. "The Contractor" gave you very bad advise. If he really is a contractor he is probably hoping to get your business after you muck it all up and call someone for help. You can do a lot of damage to your house and decrease its value and safety by trying to do dormers by yourself. Also, do you need a building permit in your town to do this? If you don't get a permit, do you think your insurance company will pay if something goes wrong? Maybe not.