I don't know your background in this but assuming it's only DIYer background, you could be in WAY over your head. There are a few matters, to begin with, that need to be considered. Soooo, based on my assumption, here goes ...
You'll need permits. Will your existing attic joists be able to support the NEW loads of your dormer? Can you even build it over the existing framing? Will you need to upgrade your HVAC unit in order to handle the increased load? Would it be less costly to move? Could you build an additions vs. a dormer? Will the value of your home actually 'diminish'? Are you prepared for cutting some interesting 'angles' in your framing timbers? Can you find the right weather to do this w/2-3 other friends over 3+ days JUST to get the roof on (so you don't damage the rest of the house?) If the 2nd floor ends up on the 1st floor, do you know how quickly your Insurance Company can spell 'incompetence'? (They won't cover any damage!)
Here's a book. How to build a dormer (I've now answered your 'question'.) However, it's almost 30 years old. Perhaps they, the publisher, or the author has a more MODERN book. What I suggest you do is hire a Pro to do the rough work to the point where you can finish the INTERIOR at your leisure. Consider hiring an Architect who has access to Engineers (should a Structural Engineer), if needed. They can also help you plan the space. IMO, it's $$$ well spent.
Sooooo, spend some of that saved $$$ on professional advice. (Remember about the Insurance Company ...) Others may differ. My best to ya and hope this helps.