This video shows the Windows Installation on a PC: downloading the program, setting it up and then deciding which Windows user accounts the rules should apply to.
That last point matters most and is the one most often overlooked. On Windows the rules are tied to a user account – if your child has an account of their own, the assignment has to be right, otherwise the time limits sit on the wrong account. Sign in as an administrator to install.
To read the steps instead, see the Windows installation guide; for questions afterwards there is the Windows FAQ. The program itself is in the download area, and the other films are in the video overview.