Measuring the Windows
You want to measure for your new windows by checking the size of the buck frame. The buck frame is the wood frame that surrounds the window. In an existing home, you may need to remove interior trim to get an accurate measurement. It is best to size the new windows so that they are 1/4 to 1/2 inch smaller than the frame. This will allow you to adjust the window as needed to make it square.
Preparing the Opening
If replacing existing windows, you will need to remove the old window. The easiest process is to cut around the outer frame of the window with an electric saw after you have removed the interior and exterior trim. Check the bottom sill of the buck frame to ensure that it is level. If the frame is severely damaged or out of square a great deal, you can replace the frame with new lumber. Use shims and a level to square the frame.
When measured properly, the new window will fit easily into the frame. Use screws through the sides of the frame to secure the window. These screws can be used to adjust the window. Shims will need to be placed along the outer frame or a tight fit. Screws can also be used on the lower and upper frame of the window to make sure that any bow in the vinyl frame is pulled out.
Finishing the Window
Once the window is in place and you have made certain that it functions properly, you will want to install insulation in any open areas between the window frame and the buck. Interior trim or drywall will need to be installed to finish the interior opening, and break metal will need to be added to the exterior to weatherproof the opening.