A couple of us got together and helped a neighbor install 6 or 7 windows one weekend. I had never done it before.
It was pretty easy, but he really is a handy guy, which helps. And, he told us how hard he worked to measure the openings accurately. He had 4 different sized windows and all of the windows he ordered fit perfectly.
It is something I would consider doing, but the ones high up on the 2nd floor were a bit tense.