I'm in the middle of building a shed and didn't think too carefully about the trim boards. I was planning on using azek (cellular PVC) for trim around a couple doors and windows. The smart siding installation instructions say to install flashing over window and door trim and leave a 3/16 gap. All the videos on youtube that show people building sheds with this stuff have them just nailing/screwing the trim in on top of the siding. Installing trim first, then cutting the smartside panels to fit around them is a pain in the a$$.

I'm assuming if i want it to last, i should install the trim first, with the flashing. I'm just worried about cutting the panels to fit exactly around a window. Any tips for ease of doing that or just trust my measurements and make sure the framing is all square.