Window Jamb Extensions board layout

I need some advice on the correct/best layout for building a window jamb extension for an unusual shaped window.

Everything I've read, for a typical square/rectangle window, the horizontal pieces are cut to the full length of the window and verticals are cut a bit short to accommodate the thickness of the board that they "stand" on.

I have a window where there are three windows installed as a single unit. The middle unit is taller than the two edges so it has the appearance of steps. Something like this
_______
_______l l________
l l
l l

My question pertains to the two corners that are the reverse direction from what is in a typically found in a square/rectangle window. Do I continue with the pattern of the vertical sides "standing" on horizontal pieces and thus extend the boards that are at the top of the shorter windows past their edge so the short verticals on the higher portion can stand on them? Or can I extend the verticals on the higher window down and attach the horizontals to the side of those pieces? I'm not sure if the typical board placement is constructed for a structural reason or an aesthetic reason and want to make sure I'm not messing it up.

 

It really doesn't matter. The sides can be long and the horizontals can fit inbetween. But if the middle window is taller, you might want to glue, miter, and pin nail those two corners together where the shorter window head piece needs to turn the corner and go up the side of the middle window.

 

I'm assuming the miter is aesthetic? I don't have a miter saw and am doing all my miter with a miter box and hand saw. I'm not a fan of doing that so if I can avoid more miter cuts I will.

 

Yes, for looks only, on those 2 miters only. It's an outside corner that will be seen from below.