When it comes to installing windows, you actually have two choices -
(A) Hire a run-of-the-mill installation agency who might know next to nothing about framing, drywall, siding, etc and turn your house upside-down for several days. You can then spend a few more days cleaning up the mess. It's not as bad as it sounds, you know, it could be fun if you have a lot of free time in hand.
(B) Or hire somebody (hint - RFI:-)) who have really streamlined the process of installing windows so much so that you can sit in the other room and practice yoga while they/we install your windows.