My $.02 says to hire this out. Ask friends, or friends of friends, for someone they personally know who can do this. You'll probably save some $$$ but to be sure of how much, DO call in a carpenter or 3 for an estimate. THEN, when this 'friend' comes by, you'll have an idea of what's involved, all the way around. AND, if he wants even more than the bids you received earlier, ask him why when XYZ can do it for less.
It's not that straight forward to instruct you on how to use the necessary tools and how to do the finish work. By cutting a hole in your roof should, right away, tell you that if you don't properly finish off the job, you'll have created a NEW problem, ..., a leak into the attic! Be sure the duct is insulated if your attic is unheated. (Maybe insulate it anyway.) I suggest you buy a fan that when you shut off the light, it stays on a little bit to push out any remaining humidity / hot-air from the bathroom. Otherwise, you could end up with moisture dripping back down into your bathroom from the vent. (This is usually accomplished with a humidity-controlled fan.) Also, if you can put the duct out to the side of the house, or even straight up to the roof where you'll have a shorter 'run', I suggest you do that too. The less duct there is, the better the duct clears out. And the fewer turns there are, the better too. Just find out how the carpenter will 'finish' the job especially if you have, say, slate for roofing or vinyl for siding. (Looks do count for something.) It may also be best to have a fan that's separate from the light switch (at least from my personal experience.)