How can I make a new basement window fit the opening?
If the window is flange mounted you have to be careful how much you trim. You need enough flange to secure the window. Most vinyl windows I have installed have a "C" shaped profile on the outside edges. If the window is secured by screws through the sides of the frame you can trim the edges. Depending on the amount to be trimmed I have used a utility knife or even a belt sander. The sander has to be used at slow speed to keep the vinyl from melting and clogging the paper. In another case I helped someone who ordered windows to large for the openings by running one edge thru my tablesaw to remove a blade width of "extra material".