Ours didn't require any cutting . There was a wood trim piece against the window frame sides on the inside and on the outside. I removed the inside trim, removed screws holding in the old window and they came right out. Ran a bead of silicone between the outside piece and the new window, put window in and shimmed it square and level. Another bead of spray foam made for windows around the edge from the inside and then put the trim back on. No more drafts blowing in all winter and it only took 20 minutes per window from start to finish.
Easiest DIY project I ever attempted.