Begin with the vertical sides cutting the bottom for length.
How to install vinyl siding trim around windows.
Cutting and installing the siding 1 place the piece of siding to be installed under the window along the bottom edge and hold it where it will be located horizontally along the wall.
Next install the casing around the sides and top of the window.
Cut the trim so it is two channel widths longer than the opening s width.
This video shows the installation of siding j channel around doors and windows.
Install the tops first then the sides and then the bottoms of the window surrounds.