Skill level required to install new windows
I don't think it's that technically hard.  It does take some time and care.  A number of years ago, I installed 21 replacement Andersen windows in my house.  I was able to do about 2 to 4 a day working part time.

Replacement Windows avoided issues that putting in a "new construction" window requires.
You have to deal with the exterior and interior finishes.  On the exterior side, its about the flashing and waterproofing. And fitting with the siding.  Inside, you have to fit the jambs to the wall, fix any insulation &amp; drywall issues, and fit the interior trim.

Even on replacement windows, I found myself having to fix and replace rotted and failing parts of the existing window sills, jambs and trim.

The job entails attention to detail, ability to cut,shim, and fit, understand the house wrap and sealing.  Eventually mounting the window level, plumb, and square.

For the unpracticed, it can be a learning curve that takes time and fiddling.  But if you are persistent you will pickup speed.   There are instructions that come with the windows, and videos you can watch online.