Install the flashing from the bottom up.
How to install vinyl siding trim around window.
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.
Install the tops first then the sides and then the bottoms of the window surrounds.
Along the tops install j channel trim with horizontal siding and base trim with vertical siding.
Install j channels at doors and windows and under roof eaves and rakes.