Then, click the link in the 'Related Articles' box at the top-center of the above web page. After you read that this is NOT the type of job for the average DIYer, let me know if it's still ANYTHING close to building a dog house. You DO need a plan! You DON'T just go cutting out a hole in the roof. Sorry, guy, but you can't be further from the truth. I can see Keith going up there and just 'cutting a hole in his roof'. What about the rafter load that's no longer supported when he 'cuts' the hole? I will give you that he ought to wear the goggles, ear plugs, and hard hat.
I'll shut up now. Keith: Sorry for this, guy. I think you'll find the above link in the 'Related Articles' box to be of use to you. There are books on this type of construction. If you need that and are willing to spend a few $$$ to save a few $$$, then post back and I'll see what I can find for you.