Along the tops install j channel trim with horizontal siding and base trim with vertical siding.
How to install vinyl siding trim around windows.
Begin with the vertical sides cutting the bottom for length.
Install j channels at doors and windows and under roof eaves and rakes.
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.
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.