A quality Window Installation contractor will use proper window shims and corner gaskets to ensure the window fits correctly.
They will measure the rough opening to check for any details that could affect the installation of the window. They will also measure the width of the bottom of the window as well as the height of the sides and center. They will use the smallest dimensions to get the most accurate measurements and make any necessary adjustments to fit. Depending on the size of the window, they may need to make further adjustments for proper fitting.
Before installing the window, the installer will check its alignment and make sure that it is plumb and level. They will then push the window frame into the rough opening until it is flush with the wall. They will use a level to make sure that the window is level and aligned. After installation, the installer will paint the window to match the rest of the exterior. When the installation is complete, they will clean the area and remove any debris. They will also dispose of the old windows and sash.